Accidentally put Ms. instead of Miss?

It is correct to use "Ms." in a couple of different situations. It is the traditional title for an unmarried woman or in the case that you do not know whether the person is married or not. Say you want to write a leader to somebody and all you have is a name like "Jane Doe", then using the title "Ms. Jane Doe" because of the uncertainty of her marriage status. You do use "Mrs." as a title for a woman who is married such as "Mr. and Mrs. John Doe." The title "Miss" is traditionally used to address a younger unmarried female including a child. You would address it to "Miss Joan Doe" (the daughter of John and Jane Doe, I guess). A title for a young male typically a child is "Master". For example, the title would be like "Master John Doe Jr." (Joan's brother).

No problem. Ms. is used when you don't know or don't care to state a marital status.