docs: update checkout action

Author: boidolr

README.md

@@ -16,7 +16,7 @@ Generate coverage.py badge like this ![coverage badge](./coverage.svg) without u
 ```yaml
 ...
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Coverage Badge
         uses: tj-actions/coverage-badge-py@v2
 ```
@@ -68,7 +68,7 @@ Support this project with a :star:
 ```yml
 ...
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
         with: 
           fetch-depth: 0