Fix ObjectDisposedException message

The call to ObjectDisposedThrowHelper.ThrowIf expects an instance or a type. This code was passing a name, which resulted in an error message resembling:

System.ObjectDisposedException : Cannot access a disposed object.
Object name: 'System.String'.

Author: drewnoakes

src/Grpc.Net.Client/GrpcChannel.cs

@@ -545,7 +545,7 @@ internal void RegisterActiveCall(IDisposable grpcCall)
         {
             // Test the disposed flag inside the lock to ensure there is no chance of a race and adding a call after dispose.
             // Note that a GrpcCall has been created but hasn't been started. The error will prevent it from starting.
-            ObjectDisposedThrowHelper.ThrowIf(Disposed, nameof(GrpcChannel));
+            ObjectDisposedThrowHelper.ThrowIf(Disposed, typeof(GrpcChannel));
 
             _activeCalls.Add(grpcCall);
         }