Update README.md to main branch rather than master

With suggestion that updates to rule to cover any
default branch.

README.md

@@ -22,7 +22,7 @@ This project's static Pages are built by [GitLab CI][ci], following the steps
 defined in [`.gitlab-ci.yml`](.gitlab-ci.yml):
 
 ```
-image: alpine:latest
+image: busybox
 
 pages:
   stage: deploy
@@ -31,8 +31,9 @@ pages:
   artifacts:
     paths:
     - public
-  only:
-  - master
+    expire_in: 1 day
+  rules:
+    - if: $CI_COMMIT_REF_NAME == $CI_DEFAULT_BRANCH
 ```
 
 The above example expects to put all your HTML files in the `public/` directory.