After speaking with many faculty members from three different schools that deal with grad apps (in preparation for my SOP writing) I would say don't worry about it. They want to choose good chemists and grammatical errors as minor as these aren't going to weigh in on that decision at all.
Now if the application hasn't been finalized just yet you can probably resubmit and overwrite the one stored in the system. The schools I am applying to allow this.