Update environment variable GOOGLE_CLOUD_KEYFILE_JSON to use `GOOGL…

…E_APPLICATION_CREDENTIALS` (GoogleCloudPlatform#3522)

Co-authored-by: Damian Myerscough <Damian.Myerscough@gmail.com>

third_party/terraform/website/docs/guides/getting_started.html.markdown

@@ -158,10 +158,10 @@ file. Name it something you can remember, and store it somewhere secure on your
 machine.
 
 You supply the key to Terraform using the environment variable
-`GOOGLE_CLOUD_KEYFILE_JSON`, setting the value to the location of the file.
+`GOOGLE_APPLICATION_CREDENTIALS`, setting the value to the location of the file.
 
 ```bash
-export GOOGLE_CLOUD_KEYFILE_JSON={{path}}
+export GOOGLE_APPLICATION_CREDENTIALS={{path}}
 ```
 
 -> Remember to add this line to a startup file such as `bash_profile` or