Update README.rst (#12)

Author: Will Myers

README.rst

@@ -281,7 +281,7 @@ Utils
 Utils houses generic helpers useful in a Button Integration.
 
 #is_webhook_authentic
-~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~~~
 
 Used to verify that requests sent to a webhook endpoint are from Button and that
 their payload can be trusted. Returns ``True`` if a webhook request body matches