t5550: fix typo in $HTTPD_URL

Commit 14111fc (git: submodule honor -c credential.* from
command line, 2016-02-29) accidentally wrote $HTTP_URL. It
happened to work because we ended up with "credential..helper",
which we treat the same as "credential.helper", applying it
to all URLs.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5550-http-fetch-dumb.sh

@@ -103,7 +103,7 @@ test_expect_success 'cmdline credential config passes into submodules' '
 	test_must_fail git clone --recursive super super-clone &&
 	rm -rf super-clone &&
 	set_askpass wrong pass@host &&
-	git -c "credential.$HTTP_URL.username=user@host" \
+	git -c "credential.$HTTPD_URL.username=user@host" \
 		clone --recursive super super-clone &&
 	expect_askpass pass user@host
 '