Fixing SendMessage API example

missing colon in the documentation between the option and the tag! 

I effectively made sure the colon is mandatory when using the WM_COPYDATA API by testing it in my [Hawkeye2](https://github.com/odalet/Hawkeye2) project makes use of this API [here](https://github.com/odalet/Hawkeye2/blob/master/src/Plugins/Hawkeye.DecompilePlugin/ilspy/ILSpyController.cs).

Hope this helps