tour: update README after App Engine runtime change

After update to Go 1.12 (in CL 198320), the local server is not needed (or even available).

Instead these new instructions will start a local instance.

Change-Id: Ie24b9c9f902ebd70d3b3b9ff8d2c4aa49be631bd
GitHub-Last-Rev: b3facf0d8dd5ceccea4c2260b5fab4f6f731e827
GitHub-Pull-Request: golang/tour#880
Reviewed-on: https://go-review.googlesource.com/c/tour/+/212999
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/README.md b/README.md
index 2477e62..1e0e248 100644
--- a/README.md
+++ b/README.md
@@ -22,10 +22,11 @@
 To run the tour server locally:
 
 ```sh
-dev_appserver.py app.yaml
+go run .
 ```
 
-and then visit [http://localhost:8080/](http://localhost:8080) in your browser.
+Your browser should now open. If not, please visit [http://localhost:3999/](http://localhost:3999).
+
 
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