Let it go. You cannot win an argument with a toddler. If she is truly trans, you'll know over the years as she is more consistent and more articulate. I know quite a few trans people and they knew from an early age that they were improperly assigned a gender.
Just call her by her name or by some neutral term of endearment (sweetie, my wonder child, cute stuff, or anything you enjoy).
You might ask yourself what it means to "accurately identify the the correct gender of other people" - why is that important, and what criteria are you using to identify people? You're probably wrong at least 10% of the time anyway, but it also focuses on people's looks and that's just a really bad road to go down with anyone, especially children. Use gender neutral terms as much as you can: "The person in the blue shirt" and "the people walking the dog." There's a person who has worked at my supermarket for 10 years at least, and I still have no idea what gender this employee is. It's okay because the person is helpful and makes great stuff in the bake shop. I've never needed to know the gender.
If she says she's 5, play along. If she says she's a pirate, play along. If she corrects you, just say, "Okay. I forgot." Don't dwell on it. Imaginative play is important anyway, and it's not necessary to constantly correct someone.