Remove horizontal padding from caption

Previously the padding had 2px padding on every side which caused
it to be 4px longer on the right than the elements, which caused
an unnecessary scrollbar on the box containing the code.
This can be solved by setting the horizontal padding to 0 so that
it only has padding above and below. You can still tell that the
element is separate from the code (presumably the original
intention of the padding) but now it doesn't cause the scrollbar
to appear.