Typo fix in docstring.

Lib/distutils/extension.py

@@ -23,8 +23,7 @@ from types import *
 class Extension:
     """Just a collection of attributes that describes an extension
     module and everything needed to build it (hopefully in a portable
-    way, but there are hooks that let you can be as unportable as you
-    need).
+    way, but there are hooks that let you be as unportable as you need).
 
     Instance attributes:
       name : string