Fix sentence & update link for GitLab CI docs

Author: LightSage

src/doc/src/guide/continuous-integration.md

@@ -47,7 +47,7 @@ rust-nightly:
 
 This will test on the stable channel and nightly channel, but any
 breakage in nightly will not fail your overall build. Please see the
-[GitLab CI](https://docs.gitlab.com/ce/ci/yaml/README.html) for more
+[GitLab CI documentation](https://docs.gitlab.com/ce/ci/yaml/index.html) for more
 information.
 
 ### builds.sr.ht