When I was your daughter's age I preferred to play with girls rather than with my own sex. However, from the age of about 7 most children prefer to form friendships exclusively with their own sex. Therefore, your daughter may soon want to start playing with girls instead of boys.
The danger is, as happened with me, if she gets frozen out from relationships with both sexes. Don't try to stop her from playing with boys: it's nice when girls and boys are able to play together. But you might wish to encourage her to try playing with girls as well so that she is used to having female friendships when the boys no longer want to play with her.