Fix docs for heading sizes

[TiGR]

Sizes here were from v3. Fixed these to correspond with v4 (for real).

Should rem sizes be also added to docs?

[schoenburg]

I was looking at this discrepancy today too and I was trying to figure out which was wrong, the docs or the code base? Seems like the sizes in the code are a bit big, but if those are correct than this is definitely a great fix.

Also agree it's a good idea to update with the rem values as well since that is how they are coded.

[outdooricon]

yes, the rems are great!

[TiGR]

Added rem sizes, not sure about if this is intended behavior (I mean the sizes). Anyway, its up to maintainers to decide.

[schoenburg]

The rems are based off of an assumed 16px value for most browsers, and is at this time actually explicitly set to that in the styles, so some of these values are a bit off.

h1 and h2 are correct.

h3, .h3 is 1.75rem so should be 28px

h4, .h4 is 1.5rem so should be 24px

h5, .h5 is 1.25rem so should be 20px

h6, .h6 is 1rem so should be 16px

[TiGR]

Not sure how I was able to mess this up - I remember calculating these numbers, but somehow ended up writing line-heights instead of these...

[mdo]

Thanks!