Update BUILDGUIDE.md (#641)

Author: johnnypham

BUILDGUIDE.md

@@ -179,10 +179,10 @@ For .NET Core, all 4 reference types are supported:
 For .NET Framework, below reference types are supported:
 
 ```bash
-> msbuild /t:BuildTestsNetCore /p:ReferenceType=Project
+> msbuild /t:BuildTestsNetFx /p:ReferenceType=Project
 # Default setting uses Project Reference.
 
-> msbuild /t:BuildTestsNetCore /p:ReferenceType=Package
+> msbuild /t:BuildTestsNetFx /p:ReferenceType=Package
 ```
 
 ### Running Tests: