Unintentionally disrespectful in the Statement of Purpose/Academic essay
The culture regarding the academic titles is quite country-specific. In some countries (e.g. Germany) academics are more often referred to by title and surname, Prof. Famous, even in a classroom. In some countries (e.g. UK) students will address their lecturers by their first name, but refer to them by their title and surname in official writing. 

However, these traditions rapidly change now because of the cultural mix in academia. With the growing numbers of professors with unusual names for European cultures, it is a more common practice now to include both name and surname after the title, e.g. Dr. Hu Rich.  It is impossible to maintain the same standard, and I don't think that academics generally care too much. It is still considered a good tone to keep the titles in official academic writing, but I don't think we expect all MSc candidates to know this. 

Overall, my advice is: don't worry to much, don't write to correct it, just make sure you do it properly next time.