EDITED TO ADD-
What I read after I posted an answer, proved my point... Most parents don't give their children credit for understanding! I know that at 6, I understood it. I know that at 6 my daughter understood it... Even at 4 she understood it.
GIVE HER CREDIT. She may surprise you and tell you that she still thinks of your husband as her dad. And that he's more special to her. She might even tell you she already understands the basic difference between a dad and a stepdad, but that it doesn't matter to her.
My daughter just turned 7. Similarly, her stepdad is more of a dad than her real dad. However, I didn't get together with my current husband until she was 4.
I would ask her why she thinks that your husband is her "real dad". Talk to her about what the differences are... But do so with tact. Let her know that it's still ok to call them both dad if that's what she wants to do. Talk to her about the difference between a "dad" and a "father". She's old enough to understand and if there's something that she doesn't quite understand, she'll tell you.
Good luck.