Update README.md

Matching readme to latest release version and fixing checkout action (best practice not to use `@main` versioning)

Author: timheuer

README.md

@@ -18,7 +18,7 @@ Basic:
 ```yaml
 steps:
 - uses: actions/checkout@main
-- uses: actions/setup-dotnet@v1
+- uses: actions/setup-dotnet@v1.5.0
   with:
     dotnet-version: '3.1.x' # SDK Version to use; x will use the latest version of the 3.1 channel
 - run: dotnet build <my project>
@@ -34,7 +34,7 @@ jobs:
         dotnet: [ '2.2.103', '3.0', '3.1.x' ]
     name: Dotnet ${{ matrix.dotnet }} sample
     steps:
-      - uses: actions/checkout@main
+      - uses: actions/checkout@v2
       - name: Setup dotnet
         uses: actions/setup-dotnet@v1
         with:
@@ -45,9 +45,9 @@ jobs:
 Authentication for nuget feeds:
 ```yaml
 steps:
-- uses: actions/checkout@main
+- uses: actions/checkout@v2
 # Authenticates packages to push to GPR
-- uses: actions/setup-dotnet@v1
+- uses: actions/setup-dotnet@v1.5.0
   with:
     dotnet-version: '3.1.x' # SDK Version to use.
     source-url: https://nuget.pkg.github.com/<owner>/index.json