Fixed wrong typo Automatically -> Atomically

According to [this comment](https://github.com/dotnet/corefx/commit/2c4cafbe7eeb4db70277b74650c9019e4d70a013#commitcomment-8817725) I accidentally 'fixed' a typo I made in the earlier pull request #150. 

This PR is intended to undo to error, unless there are other methods in GIT to undo it.