Fix typo

Author: abitmore

.github/workflows/build-and-test-ubuntu-debug.yml

@@ -8,7 +8,7 @@ jobs:
     name: Build and run tests in Debug mode
     strategy:
       matrix:
-        os: [ ubuntu-16.04, ubuntu-18.04, Ubuntu-20.04 ]
+        os: [ ubuntu-16.04, ubuntu-18.04, ubuntu-20.04 ]
     runs-on: ${{ matrix.os }}
     steps:
     - name: Install dependencies