Firstly, “fulfil” is not an error, just an acceptable alternate spelling of the word.
Secondly… I think that might partly answer the question. ;)
Thirdly and more generally though:
Edits should be significant, and 1-character edits (even to fix spelling) are most often not, which is why the help page for the full edit privilege says:
Tiny, trivial edits are discouraged — try to make the post significantly better when you edit, correcting all problems that you observe.
Free editing is a privilege earned with more reputation, the idea being that those users will have a clearer and more nuanced understanding of what kind of edits are useful to the site and what kinds aren't by the time they've earned that much rep.
That time also gives users the chance to observe existing editing practices to pick up what kinds of edits the community approves of, and which are seen as unnecessary disruption to the front page.
When looking to fix spelling, always look for other changes that would improve the post substantially. If there are none to be found, best leave it be—it's only when spelling errors riddle a post that it really starts affecting it, and at that point there are enough errors that a 6+ character edit is possible. If every other word is spelled correctly, a lone error won't adversely affect the reader's impression of the post. In such cases, its loneliness may even suggest that it's not really an error. This can especially happen with spelling variations between American English and British English, in which case it's best to avoid spelling-based edits, as they may just impose a foreign spelling standard on the original post author rather than improve the post.
It's rare for an under-6 character edit to be necessary for a post to be understood properly. Fortunately, given just a bit of time one of the higher-rep users will likely notice it and make the necessary edit. If enough time goes by with it uncorrected, it's entirely possible that it doesn't actually need changing and is considered fine how it is. Still, if it seems like a serious issue with a post is going unnoticed, you can always use a custom flag to notify a moderator to take a look.