Update checkout action reference

Author: alexcrichton

.github/workflows/main.yml

@@ -71,7 +71,7 @@ jobs:
             rust: stable
             target: x86_64-pc-windows-msvc
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@v1
       with:
         submodules: true
     - name: Install Rust (rustup)
@@ -111,7 +111,7 @@ jobs:
     name: Rustfmt
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@v1
       with:
         submodules: true
     - name: Install Rust
@@ -122,7 +122,7 @@ jobs:
     name: Publish Documentation
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@master
+      - uses: actions/checkout@v1
         with:
           submodules: true
       - name: Install Rust