Changed msg's explanation and made it shorter

Author: Manoj

Doc/library/asyncio-future.rst

@@ -196,9 +196,9 @@ Future Object
       Otherwise, change the Future's state to *cancelled*,
       schedule the callbacks, and return ``True``.
 
-      The optional argument *msg* is passed as the argument to the
+      The optional string argument *msg* is passed as the argument to the
       :exc:`CancelledError` exception raised when a cancelled Future
-      is awaited. It is typically a string.
+      is awaited.
 
       .. versionchanged:: 3.9
          Added the *msg* parameter.