Simply reset your Keyboard Dictionary.
As you type on iOS, it will begin to learn the words you use frequently. The frequently used words will take precedent after a while (most often seen with names). Sometimes this system makes a mistake and records a typo or a word that you'd rather not have iOS dish up.
To correct this matter, simply reset your Keyboard Dictionary. Tap Settings > General. Scroll down to the bottom and tap on Reset.
This will delete all the entries and restore your keyboard dictionary to its default state, removing the erroneous admission.