Bump actions/checkout from 3 to 4 in README.md (#117)

Author: tatsuyafujisaki

README.md

@@ -23,7 +23,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: dart-lang/setup-dart@v1
 
       - run: dart pub get
@@ -93,7 +93,7 @@ jobs:
         os: [ubuntu-latest, macos-latest, windows-latest]
         sdk: [3.1, stable, beta]
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: dart-lang/setup-dart@v1
         with:
           sdk: ${{ matrix.sdk }}