docs: update README.md to refer to v3

Author: wdconinc

README.md

@@ -4,15 +4,15 @@
 This GitHub Action sets up CernVM-FS for use in GitHub Workflows.
 
 ## Instructions
-You can use this GitHub Action in a workflow in your own repository with `uses: cvmfs-contrib/github-action-cvmfs@v2`.
+You can use this GitHub Action in a workflow in your own repository with `uses: cvmfs-contrib/github-action-cvmfs@v3`.
 
 A minimal job example for GitHub-hosted runners of type `ubuntu-latest`:
 ```yaml
 jobs:
   ubuntu-minimal:
     runs-on: ubuntu-latest
     steps:
-    - uses: cvmfs-contrib/github-action-cvmfs@v2
+    - uses: cvmfs-contrib/github-action-cvmfs@v3
     - name: Test CernVM-FS
       run: ls /cvmfs/sft.cern.ch/
 ```
@@ -119,7 +119,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-    - uses: cvmfs-contrib/github-action-cvmfs@v2
+    - uses: cvmfs-contrib/github-action-cvmfs@v3
     - name: Test CernVM-FS
       run: |
         ls /cvmfs/lhcb.cern.ch