Update docs.yml

Author: jamesmcroft

.github/workflows/docs.yml

@@ -25,10 +25,15 @@ jobs:
     steps:
     - uses: actions/checkout@v2
 
-    - name: Setup .NET
+    - name: Setup .NET Core 3.1
       uses: actions/setup-dotnet@v1
       with:
         dotnet-version: 3.1.x
+
+    - name: Setup .NET 5.0
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: 5.0.x
 
     - name: Setup DocFX
       uses: crazy-max/ghaction-chocolatey@v1