cmd/godoc: fix command for creating the zip file

The `-r` option (to recurse into directories) need to be specified
else the created file consists only of the specified directory and
nothing else.

Change-Id: I624184fc2e88998a3119c12f8c328603588c123c
Reviewed-on: https://go-review.googlesource.com/17283
Reviewed-by: Russ Cox <rsc@golang.org>
diff --git a/cmd/godoc/doc.go b/cmd/godoc/doc.go
index 275423a..9ff6026 100644
--- a/cmd/godoc/doc.go
+++ b/cmd/godoc/doc.go
@@ -134,7 +134,7 @@
 must be set to the .zip file directory path containing the Go root directory.
 For instance, for a .zip file created by the command:
 
-	zip go.zip $HOME/go
+	zip -r go.zip $HOME/go
 
 one may run godoc as follows: