Update README.md (#332)

Update code sample to use latest github action/checkout@v4.

Signed-off-by: Matt Seymour <mattaseymour@gmail.com>

Author: mattseymour

README.md

@@ -69,7 +69,7 @@ jobs:
 
     steps:
     # actions/checkout MUST come before auth
-    - uses: 'actions/checkout@v3'
+    - uses: 'actions/checkout@v4'
 
     - id: 'auth'
       name: 'Authenticate to Google Cloud'