SERVER-13037 Fix misspelling ("compatability" to "compatibility")

This pull request responds to an [open issue](https://jira.mongodb.org/browse/SERVER-13037) regarding a misspelled word in an error message given by the MongoDB shell. I also corrected two other instances of the mistake which appeared in comments. The "issue" is trivial, but addressing it does add a little polish to MongoDB. All tests still pass on my personal machine, a MacBook Air running Mavericks (OS X 10.9.2), but I have only tested with the "origin/master" branch from GitHub. I'm not sure whether new tests are really necessary for this sort of thing, so I haven't written any yet.