doc: Update env variable mentioned in Authoring Guide (GoogleCloudPlatform#7406)

The correct value to be changed in `testing/test-env.sh` is `GOOGLE_CLOUD_PROJECT`
instead of `GCLOUD_PROJECT`.

Source:

https://github.com/GoogleCloudPlatform/python-docs-samples/blob/1d6736bd49778b2440e348890ec444d785c4fb60/testing/test-env.tmpl.sh#L16

Author: Mariatta

AUTHORING_GUIDE.md

@@ -640,7 +640,7 @@ We suggest that you copy this file as follows:
 
 ```sh
 $ cp testing/test-env.tmpl.sh testing/test-env.sh
-$ editor testing/test-env.sh  # change the value of `GCLOUD_PROJECT`.
+$ editor testing/test-env.sh  # change the value of `GOOGLE_CLOUD_PROJECT`.
 ```
 
 You can easily `source` this file for exporting the environment variables.