Add _REGION substitute to readme cloud build section

Signed-off-by: KeisukeYamashita <[email protected]>

Author: KeisukeYamashita

README.md

@@ -396,6 +396,7 @@ GitHub private repositories require the `repo` and `write:discussion` permission
   - `BUILD_ID`
   - `PROJECT_ID`
   - `_PR_NUMBER`
+  - `_REGION`
 - Recommended trigger events
   - `terraform plan`: Pull request
   - `terraform apply`: Push to branch