Fix docstring for _migrate_volume_completion

"Migrate a volume to the specified host." as the description of
_migrate_volume_completion func is incorrect.

Closes-Bug: #1253910
Change-Id: Ica944d9a7fa962ff56a9f0107ac1519623d2e551

cinder/api/contrib/admin_actions.py

@@ -163,7 +163,7 @@ def _migrate_volume(self, req, id, body):
 
     @wsgi.action('os-migrate_volume_completion')
     def _migrate_volume_completion(self, req, id, body):
-        """Migrate a volume to the specified host."""
+        """Complete an in-progress migration."""
         context = req.environ['cinder.context']
         self.authorize(context, 'migrate_volume_completion')
         try: