<p>In general you should not fix a single letter typo; one exception would be if it's an important word in the title (since people search for those!). The fact that the system tries to prevent you from making very small edits is deliberate -- it tries to stop you because you probably shouldn't. The <a href="https://stats.stackexchange.com/help/privileges/edit">editing-privilege page</a> says "Tiny, trivial edits are discouraged - try to make the post significantly better when you edit, correcting all problems that you observe." ... single letter edits are about as tiny as they come, so that's what it's asking you to avoid.</p>

<p>I posted some of those links in comments in order that you saw some of the discussion of edge cases and potential exceptions (e.g. many one-character changes in mathematics may be the opposite of trivial; the abovementioned issue with title keywords)</p>
