Wrap cookbook install snippet only in <code> tags.

This adjusts the cookbook install snippet to only wrap it in <code> tags with a
class to style it as expected. This fixes a regression introduced in
8874fa78ee86fca0b4754c12da50684b0d1bf71b
Merge pull request #574 from opscode/code_pre_style_adjustments

Wrap cookbook install snippet only in <code> tags.