Update README.md

Removed 3.1.x dotnet requirement from Sample.

Author: sukhans

README.md

@@ -31,7 +31,6 @@ steps:
 - uses: actions/setup-dotnet@v1
   with:
     dotnet-version: |
-      3.1.x
       5.0.x
       6.0.x
 - name: Run Microsoft Security DevOps