[openstreetmap-website] Fix overlapping text in markdown help (#273)

[Leo Koppelkamm]

This also fixes a problem where the helpbox doesn't reflow on smaller screens:
![photo](https://f.cloud.github.com/assets/183302/493267/a9ac59d6-bb2f-11e2-964f-8d2b2d0540d0.jpg)


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Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/openstreetmap-website/pull/273#issuecomment-17782380
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