content: fix formatting on appengine-specific slides

The trailing spaces on empty 'appengine:' lines were removed at some
point, but those spaces are actually critical to making the formatting
work.

Take a look at https://tour.golang.org/welcome/3 to see how it's broken;
there are no line breaks or code block formatting.

The algorithm for transforming the 'appengine:' blocks could be
improved, but the easy fix right now is to restore the trailing spaces.

Change-Id: I1d2ba085e4b09099b073a521ca20e4333e69bbe9
Reviewed-on: https://go-review.googlesource.com/c/tour/+/165477
Reviewed-by: Andrew Bonventre <andybons@golang.org>
diff --git a/content/basics.article b/content/basics.article
index 082a6ab..e124e1a 100644
--- a/content/basics.article
+++ b/content/basics.article
@@ -17,7 +17,7 @@
 #appengine: *Note:* The environment in which these programs are executed is
 #appengine: deterministic, so each time you run the example program
 #appengine: `rand.Intn` will return the same number.
-#appengine:
+#appengine: 
 #appengine: (To see a different number, seed the number generator; see [[https://golang.org/pkg/math/rand/#Seed][`rand.Seed`]].
 #appengine: Time is constant in the playground, so you will need to use something else as the seed.)
 
diff --git a/content/welcome.article b/content/welcome.article
index fe512e7..7b318a7 100644
--- a/content/welcome.article
+++ b/content/welcome.article
@@ -70,41 +70,41 @@
 Click the [[javascript:highlightAndClick(".next-page")]["next"]] button or type `PageDown` to continue.
 
 #appengine: * Go offline
-#appengine:
+#appengine: 
 #appengine: This tour is also available as a stand-alone program that you can use
 #appengine: without access to the internet.
-#appengine:
+#appengine: 
 #appengine: The stand-alone tour is faster, as it builds and runs the code samples
 #appengine: on your own machine.
-#appengine:
+#appengine: 
 #appengine: To run the tour locally install and run the tour binary:
-#appengine:
+#appengine: 
 #appengine:   go get golang.org/x/tour
 #appengine:   tour
-#appengine:
+#appengine: 
 #appengine: The tour program will open a web browser displaying
 #appengine: your local version of the tour.
-#appengine:
+#appengine: 
 #appengine: Or, of course, you can continue to take the tour through this web site.
 
 #appengine: * The Go Playground
-#appengine:
+#appengine: 
 #appengine: This tour is built atop the [[https://play.golang.org/][Go Playground]], a
 #appengine: web service that runs on [[https://golang.org/][golang.org]]'s servers.
-#appengine:
+#appengine: 
 #appengine: The service receives a Go program, compiles, links, and runs the program inside
 #appengine: a sandbox, then returns the output.
-#appengine:
+#appengine: 
 #appengine: There are limitations to the programs that can be run in the playground:
-#appengine:
+#appengine: 
 #appengine: - In the playground the time begins at 2009-11-10 23:00:00 UTC (determining the significance of this date is an exercise for the reader). This makes it easier to cache programs by giving them deterministic output.
-#appengine:
+#appengine: 
 #appengine: - There are also limits on execution time and on CPU and memory usage, and the program cannot access external network hosts.
-#appengine:
+#appengine: 
 #appengine: The playground uses the latest stable release of Go.
-#appengine:
+#appengine: 
 #appengine: Read "[[https://blog.golang.org/playground][Inside the Go Playground]]" to learn more.
-#appengine:
+#appengine: 
 #appengine: .play welcome/sandbox.go
 
 * Congratulations