Fix typo.

Doc/lib/libstdtypes.tex

@@ -1107,7 +1107,7 @@ Notes:
      
   \var{key} specifies a function of one argument that is used to
      extract a comparison key from each list element:
-     \samp{\var{cmp}=\function{str.lower}}
+     \samp{\var{key}=\function{str.lower}}
 
   \var{reverse} is a boolean value.  If set to \code{True}, then the
      list elements are sorted as if each comparison were reversed.