t5531: avoid using an abbreviated option

It was probably just an oversight: the `--recurse-submodules` option
puts the term "submodules" in the plural form, not the singular one.

To avoid future problems in case that another option is introduced that
starts with the prefix `--recurse-submodule`, let's just fix this.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5531-deep-submodule-push.sh

@@ -363,7 +363,7 @@ test_expect_success 'push succeeds if submodule has no remote and is on the firs
 		) &&
 		git add b &&
 		git commit -m "added submodule" &&
-		git push --recurse-submodule=check origin master
+		git push --recurse-submodules=check origin master
 	)
 '