GH Actions: version update for actions/checkout

Ref: https://github.com/actions/checkout/releases

.github/workflows/docs.yaml

@@ -10,7 +10,7 @@ jobs:
     if: github.repository == 'PHPMailer/PHPMailer'
     steps:
       - name: Checkout sources
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
         with:
           fetch-depth: 1
       - name: Build Docs