Documentation: CI: GitLab Pages Example

Replace 'only' with 'rules'.
'only' is not actively developed anymore with GitLab (see https://docs.gitlab.com/ee/ci/yaml/#only--except)

guide/src/continuous-integration.md

@@ -141,8 +141,8 @@ pages:
     - mdbook --version || cargo install mdbook
   script:
     - mdbook build -d public
-  only:
-      - master 
+  rules:
+    - if: '$CI_COMMIT_REF_NAME == "master"'
   artifacts:
     paths:
       - public