Improve clarity of specificity section

[karlhorky]

Hi Amelia! Reading through the specificity section with my "beginner programmer" hat on, I was confused a bit by some of the language around "hits" and what the four levels actually mean.

I added some explanations and reworked some of the language (while trying to keep the narrative), and I think it comes over a bit more clearly:

• [x] Add a sentence about how specificity is calculated
• [x] Fix error:

  -html property
  +html attribute
  

• [x] Change language from hits to match (ex. selector has three hits to selector matches three times)
• [x] Change language from four levels to some levels, to allow easily mentioning the three levels of selector specificity later, without confusing things ("there were four levels before, so these three must be in addition to the previous four"). Not entirely sold on this change though - not as elegant as I would like.

I'm open for feedback though, in case you think that there could be a better version.