README: bump version of actions/checkout in examples

Author: jhenstridge

README.md

@@ -14,7 +14,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - uses: snapcore/action-build@v1
       id: build
     - uses: snapcore/action-publish@v1
@@ -31,7 +31,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - uses: snapcore/action-build@v1
       id: build
     - uses: snapcore/action-publish@v1