The purpose of a subject line (more common than "title," in my experience) is to enable the reader to find out what a message is about in an instant without having to read it. Yours does this job very well. I wish more people were this specific: I often get e-mails with subject lines like "Good morning," "Question," or the name of the sender! These are completely useless.
The only thing you could possibly add in the future is a one-word indication of what you want your boss to approve. "Request for travel approval," "Request for budget approval," "Request for schedule approval," etc., are more specific while still being short enough for the purpose.
On that same note, please keep in mind the subject of a thread when you open it. "Email title" will not help anyone else who is wondering if "Request For Approval" is a good email title and decides to search through our threads for an answer.