Update issue-791.yml

Author: priyagupta108

.github/workflows/issue-791.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ${{ matrix.operating-system }}
     strategy:
       matrix:
-        operating-system: [ubuntu-18.04, ubuntu-16.04]
+        operating-system: [ubuntu-20.04, ubuntu-18.04, ubuntu-16.04]
 
     steps:
     - name: Checkout Repository