Switch to checkout v3 to remove node12 warning

Author: a-grealish

.github/workflows/test.yml

@@ -12,7 +12,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
       - name: Run LinkChecker
         id: self-check
         uses: ./

README.md

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
       - name: Check URLs
         uses: paramt/url-checker@master
         with: