Slight documentation tweak to reflect current command

Thanks for adopting the plugin! I'm excited to use it as a quick way to post to Twitter.

The documentation lists 'Send Tweet' command while the active command as of v1.0.2 updated 4/12/13 (per QS Plugin interface).

Since the actual command is 'Tweet', I changed it in the documentation.

Hope this helps someone else like me who had trouble figuring out the correct command.