Added @ref in uses step of gh action

Without the ref the action would throw an error.

Thank you for this nice action

README.md

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v1
-      - uses: benjlevesque/short-sha
+      - uses: benjlevesque/short-sha@v.1.1
         id: short-sha
         with:
           length: 6