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, but I wouldn't use those on an edit such as this. Your edit will most likely get rejected.

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 the original poster or one moderator which have binding votes). 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, but I wouldn't use those on an edit such as this. Your edit will most likely get rejected.