Update actions/checkout to current v3

Author: Rotzbua

README.md

@@ -120,7 +120,7 @@ jobs:
   lint:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: arduino/arduino-lint-action@v1
 ```
 
@@ -132,7 +132,7 @@ jobs:
   lint:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: arduino/arduino-lint-action@v1
         with:
           library-manager: update