Change submodule URL to use HTTPS rather than SSH

This allows users who don't have an account on git.fsfe.org to fetch the
submodule. Authenticated users can set a local URL for the submodule if
they wish to push directly to the submodule from this repository:

git config submodule.api-worker.url [email protected]:reuse/api-worker.git

Author: seabass-labrax

.gitmodules

@@ -3,4 +3,4 @@
 	url = https://git.fsfe.org/fsfe-system-hackers/forms.git
 [submodule "api-worker"]
 	path = api-worker
-	url = git@git.fsfe.org:reuse/api-worker.git
+	url = https://git.fsfe.org/reuse/api-worker.git