When someone's code is wrong, usually the thinking that goes with it is wrong too. So add an answer of your own with the correct thinking and the correct code.
If the thinking is right and the person just made a typo, you can:
- go earn enough rep to comment then add a comment
- wait for someone else to spot it and fix it for you - people with editing privileges who don't need to use up the time of reviewers
- if the error is so subtle that few will spot it, and the asker using it would somehow be terrible, and the views on this question are so low that perhaps no-one else will be able to help this situation in time, then as a last resort you can suggest an edit.
First, look for anything else you can change that genuinely needs changing (spelling, grammar, punctuation, vague or unclear wording.) You are likely to find some if this is a genuine "typo" since a person in a hurry often makes more than one mistake. In fact if you don't find any that increases the chances this isn't a typo but is a deliberate choice by the OP, meaning you should be writing your own answer.
However, should you have genuinely come across that one-in-a-million post that must be fixed RIGHT NOW by you and has nothing else wrong with it, I suppose you can. In that case, do something to use up the 6 character limit such as adding a comment explaining your change
//forgot semi colon on next line
x = 0;
Someone else will probably remove your "useless characters" later. However this is really not the best choice, the best choice is simply to answer the question correctly yourself, which should also gain you the rep you need. Typo edits are rarely so urgent that they must be performed by someone who can only suggest them - in time, someone with full privileges will come along and take care of it, or someone with enough rep to comment will notice and leave a comment for the author, who can do the edit in response.
See also http://meta.stackexchange.com/a/94358/147247