Documentation: minor cleanup in a use case in 'git stash' manual

There is no need to explicitly pass the file to be committed to 'git
commit', because it's contents is already in the index.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-stash.txt

@@ -207,7 +207,7 @@ each change before committing:
 $ git add --patch foo            # add just first part to the index
 $ git stash save --keep-index    # save all other changes to the stash
 $ edit/build/test first part
-$ git commit foo -m 'First part' # commit fully tested change
+$ git commit -m 'First part'     # commit fully tested change
 $ git stash pop                  # prepare to work on all other changes
 # ... repeat above five steps until one commit remains ...
 $ edit/build/test remaining parts