Update br-2.html

In your Pluralsight course (the "A Fluid Foundation" module), br-2.html starts with a body width of 90% and this allows the width to dynamically change with the size of the browser.  In the version I cloned, the body width was still set to a static 300px which confused me for a minute until I reviewed the code and saw the difference.  I think it'd be nice to have this match the course.