Merge pull request #906 from practicalswift/typo-fixes-20160107b

[gardening] Fix typo introduced a couple of hours ago :-)

Author: nkcsgexi

docs/LibraryEvolution.rst

@@ -663,7 +663,7 @@ members may always be removed from protocol extensions.
 .. admonition:: TODO
 
     We don't have an implementation model hammered out for adding new
-    defaulted requirements, but it is desireable.
+    defaulted requirements, but it is desirable.
 
 
 Classes