Edit description a bit

Doc/lib/libitertools.tex

@@ -132,9 +132,9 @@ by functions or loops that truncate the stream.
 
 \begin{funcdesc}{groupby}{iterable\optional{, key}}
   Make an iterator that returns consecutive keys and groups from the
-  \var{iterable}.  \var{key} is function computing a key value for each
+  \var{iterable}.  \var{key} is a function computing a key value for each
   element.  If not specified or is \code{None}, \var{key} defaults to an
-  identity function   (returning the element unchanged).  Generally, the
+  identity function and returns  the element unchanged.  Generally, the
   iterable needs to already be sorted on the same key function.
 
   The returned group is itself an iterator that shares the underlying