[x/tour] tour: update README after App Engine runtime change
[x/tour] 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 <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
X-Tour-Commit: 59af52b46224c65cf71b551cca89922747a86e61

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