There is an autocorrect feature in FD13, so you'd need to remove "i" from that autocorrect list. To do so, go to Final Draft 13 in the top left of your screen in the menu bar, click Settings, and select Autocorrect from the top tabs:
In that list, scroll until you get to "i," click on it to highlight it, then click the Minus button, like so:
Then click OK.
"i" should no longer be corrected to "I."