Fix typographical error(s)

[orthographic-pedant]

@Tyrsis, I've corrected a typographical error in the documentation of the EssentialsPlugin project. Specifically, I've changed certian to certain. You should be able to merge this pull request automatically. However, if this was intentional or if you enjoy living in linguistic squalor, please let me know and create an issue on my home repository.

[McyD]

As far as I know this version isn't being maintained any more. Current versions are at https://github.com/SEServerExtender/EssentialsPlugin