Whoa is the Correct Spelling
An Urban Dictionary entry for the wrong spelling "woah" submitted by Grammar Slammer says, ""Woah, I have nothing better to do with my time than to look up the misspelled word 'woah'."
A Slate article calls whoa the "Internet's most variously spelled word." They note whoa spelling errors made by major media outlets. There are also products being sold using incorrect spellings of the word.
Why is it Misspelled So Frequently?
The word is frequently misspelled on social media. The main reason is that it is an easy word to misspell if you are typing fast and not thinking. A Twitter search for "woah" shows new typos made almost hourly in frequency. A meme called the Crash Bandicoot "Woah" meme also put the wrong spelling out there in the universe. You can find out more about the meme on Know Your Meme.
The song is also misspelled in a lot of song lyrics. Lyrics.com provides over 3,000 results for the use of "woah." Watysky got it right in their song, "Whoa Whoa Whoa."
How to Remember the Correct Spelling
There is a great trick for remembering the correct spelling of "whoa" on Grammarly. Just remember that "whoa" has the question word "who" in it.