Documentation/git-push: fix placeholder formatting

Format the placeholder as monospace to match other occurrences in this
file and obey CodingGuidelines.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-push.txt

@@ -198,7 +198,7 @@ branch we have for it.
 +
 `--force-with-lease=<refname>:<expect>` will protect the named ref (alone),
 if it is going to be updated, by requiring its current value to be
-the same as the specified value <expect> (which is allowed to be
+the same as the specified value `<expect>` (which is allowed to be
 different from the remote-tracking branch we have for the refname,
 or we do not even have to have such a remote-tracking branch when
 this form is used).