Updating readme snippets to show latest SDK

Given the LTS release of .NET Core it's good practice that any samples using the SDK should use the supported versions.  2.2 is no longer a supported version in a month or so and the latest SDK should be used in examples.  Minor change, but important for copy/paste folks.

Author: timheuer

README.md

@@ -20,7 +20,7 @@ steps:
 - uses: actions/checkout@master
 - uses: actions/setup-dotnet@v1
   with:
-    dotnet-version: '2.2.103' # SDK Version to use.
+    dotnet-version: '3.1.100' # SDK Version to use.
 - run: dotnet build <my project>
 ```
 
@@ -31,7 +31,7 @@ jobs:
     runs-on: ubuntu-16.04
     strategy:
       matrix:
-        dotnet: [ '2.2.103', '3.0.100', '3.1.100-preview1-014459' ]
+        dotnet: [ '2.2.103', '3.0.100', '3.1.100' ]
     name: Dotnet ${{ matrix.dotnet }} sample
     steps:
       - uses: actions/checkout@master
@@ -49,7 +49,7 @@ steps:
 # Authenticates packages to push to GPR
 - uses: actions/setup-dotnet@v1
   with:
-    dotnet-version: '2.2.103' # SDK Version to use.
+    dotnet-version: '3.1.100' # SDK Version to use.
     source-url: https://nuget.pkg.github.com/<owner>/index.json
   env:
     NUGET_AUTH_TOKEN: ${{secrets.GITHUB_TOKEN}}