updated the interop spec to conform with recent name change

doc/interop-test-descriptions.md

@@ -779,21 +779,21 @@ Client asserts:
 
 ### unimplemented_method
 
-This test verifies that calling an unimplemented RPC method returns the 
+This test verifies that calling an unimplemented RPC method returns the
 UNIMPLEMENTED status code.
 
 Server features:
 N/A
 
 Procedure:
-* Client calls `grpc.testing.TestService/UnimplementedMethod` with an empty
+* Client calls `grpc.testing.TestService/UnimplementedCall` with an empty
   request (defined as `grpc.testing.Empty`):
 
     ```
     {
     }
     ```
-   
+
 Client asserts:
 * received status code is 12 (UNIMPLEMENTED)