checklist: fix links (#4285)

Fix a couple of broken links in the MSC checklist

Author: richvdh

MSC_CHECKLIST.md

@@ -43,9 +43,9 @@ clarification of any of these points.
   - [ ] Alternatives
   - [ ] Dependencies
 - [ ] Stable identifiers are used throughout the proposal, except for the unstable prefix section
-  - [ ] Unstable prefixes [consider](README.md#unstable-prefixes) the awkward accepted-but-not-merged state
+  - [ ] Unstable prefixes [consider](/README.md#unstable-prefixes) the awkward accepted-but-not-merged state
   - [ ] Chosen unstable prefixes do not pollute any global namespace (use “org.matrix.mscXXXX”, not “org.matrix”).
-- [ ] Changes have applicable [Sign Off](CONTRIBUTING.md#sign-off) from all authors/editors/contributors
+- [ ] Changes have applicable [Sign Off](/CONTRIBUTING.md#sign-off) from all authors/editors/contributors
 - [ ] There is a dedicated "Security Considerations" section which detail
   any possible attacks/vulnerabilities this proposal may introduce, even if this is "None.".
   See [RFC3552](https://datatracker.ietf.org/doc/html/rfc3552) for things to think about,