checkout: accord documentation to what git does

Signed-off-by: NNicolas Sebrecht <nicolas.s.dev@gmx.fr>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-checkout.txt

@@ -238,7 +238,7 @@ the above checkout would fail like this:
 +
 ------------
 $ git checkout mytopic
-fatal: Entry 'frotz' not uptodate. Cannot merge.
+error: You have local changes to 'frotz'; not switching branches.
 ------------
 +
 You can give the `-m` flag to the command, which would try a