docs: fix formatting of rev-parse's --show-superproject-working-tree

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rev-parse.txt

@@ -261,7 +261,7 @@ print a message to stderr and exit with nonzero status.
 --show-toplevel::
 	Show the absolute path of the top-level directory.
 
---show-superproject-working-tree
+--show-superproject-working-tree::
 	Show the absolute path of the root of the superproject's
 	working tree (if exists) that uses the current repository as
 	its submodule.  Outputs nothing if the current repository is