Adjust region tag (#4575)

Adjust region tag to match the quickstart v2 samples in other languages

- [X] Please **merge** this PR for me once it is approved.

Author: melaniedejong

iam/api-client/quickstartv2.py

@@ -14,7 +14,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-# [START iam_quickstartv2]
+# [START iam_quickstart_v2]
 import os
 
 from google.oauth2 import service_account
@@ -121,4 +121,4 @@ def set_policy(crm_service, project_id, policy):
     # TODO: Replace with the ID of your member in the form 'user:[email protected].
     member = "your_member"
     quickstart(project_id, member)
-# [END iam_quickstartv2]
+# [END iam_quickstart_v2]