Fix typo.

Doc/ref/ref3.tex

@@ -450,7 +450,7 @@ Special attributes:
   \lineiii{__module__}{The name of the module the function was defined
     in, or \code{None} if unavailable.}{Writable}
 
-  \lineiii{func_defaults}{Atuple containing default argument values
+  \lineiii{func_defaults}{A tuple containing default argument values
     for those arguments that have defaults, or \code{None} if no
     arguments have a default value}{Writable}