Run ci on the master branch (#29)

The CI workflow has always intended to run when the master branch
changes, however the workflow definition is targeting the wrong branch
name.

With this change, CI should run on the head commits of all PRs, AND the
master branch.

.github/workflows/ci.yaml

@@ -3,7 +3,7 @@ name: CI
 on:
   push:
     branches:
-      - main
+      - master
   pull_request:
 
 jobs: