fix: docs issue template was busted

Apparently forgejo has a more creative interpretation of \(\) than I was
hoping in their markdown parser and thought it was maths. I have no idea
then how you put a link in parens next to another square-bracket link,
but I am not going to worry about it.

There were several more typos, which I also fixed.

Fixes: #517
Change-Id: I6b144c6881f92ca60ba72a304ce7a0bcb9c6659a

.github/ISSUE_TEMPLATE/missing_documentation.md

@@ -2,7 +2,7 @@
 name: Missing or incorrect documentation
 about: Help us improve the reference manual
 title: ''
-labels: documentation
+labels: docs
 assignees: ''
 
 ---
@@ -19,10 +19,10 @@ assignees: ''
 
 <!-- make sure this issue is not redundant or obsolete -->
 
-- [ ] checked [latest Lix manual] \([source]\)
+- [ ] checked [latest Lix manual] or its [source code]
 - [ ] checked [documentation issues] and [recent documentation changes] for possible duplicates
 
-[latest Nix manual]: https://docs.lix.systems/manual/lix/nightly
-[source]: https://git.lix.systems/lix-project/lix/src/main/doc/manual/src
+[latest Lix manual]: https://docs.lix.systems/manual/lix/nightly
+[source code]: https://git.lix.systems/lix-project/lix/src/main/doc/manual/src
 [documentation issues]: https://git.lix.systems/lix-project/lix/issues?labels=151&state=all
 [recent documentation changes]: https://gerrit.lix.systems/q/p:lix+path:%22%5Edoc/manual/.*%22