Misspelled Boss's Name on Email

I’m so nervous about my first day at work that I didn’t pay attention while typing an email to my boss introducing myself. Instead of signing off as Sincerely [My First Initial]M, it read Sincerely [My First Initial]B... The funny thing is, her name starts with B! When she asked if there had been a mix-up, I almost choked on the explanation. I guess I’m still getting used to this new place...