Fixed variable name (#4619)

Author: crimsonred

content/actions/guides/building-and-testing-net.md

@@ -48,7 +48,7 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - name: Setup .NET Core SDK ${{ matrix.dotnet }}
+    - name: Setup .NET Core SDK ${{ matrix.dotnet-version }}
       uses: actions/setup-dotnet@v1.7.2
       with:
         dotnet-version: ${{ matrix.dotnet-version }}