Small tweaks to the README example

Author: eregon

README.md

@@ -82,9 +82,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu, macos]
-        ruby: [2.5, 2.6, 2.7, head, debug, jruby, jruby-head, truffleruby, truffleruby-head]
-    runs-on: ${{ matrix.os }}-latest
+        os: [ubuntu-latest, macos-latest]
+        ruby: [2.5, 2.6, 2.7, head, jruby, jruby-head, truffleruby, truffleruby-head]
+    runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v2
     - uses: ruby/setup-ruby@v1