Added missing "$" to github actions yaml

Author: seanlaw

.github/workflows/github-actions.yml

@@ -2,7 +2,7 @@ name: github-actions
 on: [push, pull_request]
 jobs:
   unit-testing:
-    runs-on: {{ matrix.os }}
+    runs-on: ${{ matrix.os }}
       strategy:
         matrix:
           os: [ubuntu-latest, macos-latest, windows-latest]