Fix double br issue.

Did you notice that some problem statement HTML show double line breaks as opposed to the arena in which they look fine? I have spend a long time wondering what causes this, I finally found a fix: Replace `<br></br>` with `<br/>`. The reason is that arena uses XHTML, so <br> tags are some times closed: `<br></br>`. But this is interpreted as two BR tags in browsers in HTML mode.