Fix typo introduced a couple of hours ago :-)

practicalswift · practicalswift · commit d6b2786302e4 · 2016-01-07T21:40:44.000+01:00
diff --git a/docs/LibraryEvolution.rst b/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
