Update to checkout action v3

.github/workflows/mdbook.yml

@@ -12,7 +12,7 @@ jobs:
     concurrency:
       group: ${{ github.workflow }}-${{ github.ref }}
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Setup mdBook
         uses: peaceiris/actions-mdbook@v1

.github/workflows/tests.yml

@@ -20,7 +20,7 @@ jobs:
         rust: [stable]
 
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@v3
 
     - name: Install ${{ matrix.rust }}
       uses: actions-rs/toolchain@v1
@@ -47,7 +47,7 @@ jobs:
     name: Checking fmt and docs
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@v3
     - uses: actions-rs/toolchain@v1
       with:
         toolchain: stable