Fix masthead paragraph margin from overflowing

Masthead paragraph was going under masthead image in
https://developer.mozilla.org/en-US/docs/Mozilla/Firefox_OS. This patch
fixes the behaviour.

Remove masthead paragraph right margin from tablets.

Before: http://i.imgur.com/p4AtXGu.png
After: http://i.imgur.com/XD0oMba.png

Signed-off-by: Kaustav Das Modak <kaustav.dasmodak@yahoo.co.in>