Fix typos in documentation.

Hello! I just found your very nice plugin. I have developed a similar one ([SwapText plugin](http://www.vim.org/scripts/script.php?script_id=4971); just released on vim.org) that works by deleting, and only then invoking a custom operator. I think they can actually complement each other.

While trying out yours, I noticed some minor issues. I hope you'll accept these pull requests, and keep on maintaining this!

-- cheers, ingo