Update example to use checkout@v2. (#24)

Signed-off-by: Naseem <naseem@transit.app>

Author: naseemkullah

README.md

@@ -37,7 +37,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
+
+      - name: Fetch history
+        run: git fetch --prune --unshallow
 
       - name: Run chart-testing (lint)
         id: lint