Fix a couple of typos/spelling errors in the documentation (authtication, commandline, is is, whishlist)

Thanks for creating this nifty plugin! I don't have to use it often, but it oh so handy when you really need it.

Here's a few typo fixes I discovered when reading the docs. Not all instances of 'authtication' was fixed by #28 it seems, but now it should be resolved. 

Might be worth looking into a solution that doesn't seperate `doc/SudoEdit.txt` and `README` though, or automaticaly keeps them identical. I haven't written a plugin before nor a vimball so I won't comment on how it should be done.