Fix docfx build
Fix docfx build

Looks like older versions of docfx didn't like .NET Core SDK 3.1 (or
minor updates to it)

jskeet/google-api-dotnet-client,jskeet/google-api-dotnet-client,googleapis/google-api-dotnet-client,jskeet/google-api-dotnet-client,googleapis/google-api-dotnet-client,googleapis/google-api-dotnet-client