docs: Fix typo in gitlab callback url in dev setup instructions.
docs: Fix typo in gitlab callback url in dev setup instructions.

The endpoint is /complete/gitlab/ - with the slash.

rht/zulip,andersk/zulip,zulip/zulip,zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip,andersk/zulip,rht/zulip,andersk/zulip,zulip/zulip,andersk/zulip,zulip/zulip,zulip/zulip,rht/zulip,andersk/zulip,rht/zulip,rht/zulip,zulip/zulip,rht/zulip,rht/zulip