Fix typo in positional method docstring

Author: p13i

googleapiclient/_helpers.py

@@ -40,7 +40,7 @@
 
 
 def positional(max_positional_args):
-    """A decorator to declare that only the first N arguments my be positional.
+    """A decorator to declare that only the first N arguments may be positional.
 
     This decorator makes it easy to support Python 3 style keyword-only
     parameters. For example, in Python 3 it is possible to write::