> How can I edit only one character?  

Short answer: You don't.  

Long answer: The 6 character requirement has a purpose. Every edit a sub 2k user makes has to be reviewed by at least two normal users (or one moderator). Removing one character of whitespace isn't an efficient edit (Are you *sure* there was nothing else to improve?) and wastes reviewer time (Is it really that big of a deal?).  

If you find a truly important edit that doesn't make the 6 character limit you can follow any of the steps [outlined in this answer](http://meta.askubuntu.com/a/11664/44179), but I wouldn't use those on an edit such as this. Your edit will most likely get rejected.