[AppService] fixing flag cited in warning message (#12364)

src/azure-cli/azure/cli/command_modules/appservice/custom.py

@@ -2342,7 +2342,7 @@ def create_function(cmd, resource_group_name, name, storage_account, plan=None,
     # pylint: disable=too-many-statements, too-many-branches
     if functions_version is None:
         logger.warning("No functions version specified so defaulting to 2. In the future, specifying a version will "
-                       "be required. To create a 2.x function you would pass in the flag `--functions_version 2`")
+                       "be required. To create a 2.x function you would pass in the flag `--functions-version 2`")
         functions_version = '2'
     if deployment_source_url and deployment_local_git:
         raise CLIError('usage error: --deployment-source-url <url> | --deployment-local-git')