Update region tags (GoogleCloudPlatform#4574)

## Description

Fixes #<ISSUE-NUMBER>

Note: It's a good idea to open an issue first for discussion.

## Checklist
- [ ] I have followed [Sample Guidelines from AUTHORING_GUIDE.MD](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md)
- [ ] README is updated to include [all relevant information](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#readme-file)
- [ ] **Tests** pass:   `nox -s py-3.6` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
- [ ] **Lint** pass:   `nox -s lint` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
- [ ] These samples need a new **API enabled** in testing projects to pass (let us know which ones)
- [ ] These samples need a new/updated **env vars** in testing projects set to pass (let us know which ones)
- [ ] Please **merge** this PR for me once it is approved.
- [ ] This sample adds a new sample directory, and I updated the [CODEOWNERS file](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/.github/CODEOWNERS) with the codeowners for this sample

Author: averikitsch

run/helloworld/Dockerfile

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-# [START run_hello_dockerfile]
+# [START run_helloworld_dockerfile]
 
 # Use the official lightweight Python image.
 # https://hub.docker.com/_/python
@@ -35,4 +35,4 @@ RUN pip install Flask gunicorn
 # to be equal to the cores available.
 CMD exec gunicorn --bind :$PORT --workers 1 --threads 8 --timeout 0 main:app
 
-# [END run_hello_dockerfile]
+# [END run_helloworld_dockerfile]

run/helloworld/main.py

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-# [START run_hello_service]
+# [START run_helloworld_service]
 import os
 
 from flask import Flask
@@ -28,4 +28,4 @@ def hello_world():
 
 if __name__ == "__main__":
     app.run(debug=True, host='0.0.0.0', port=int(os.environ.get('PORT', 8080)))
-# [END run_hello_service]
+# [END run_helloworld_service]