Revert "monospace, monospace" change accidentally made in 45cc401

I usually watch all the changes pretty closely in normalize.css.

It looks like during the code clean-up of 45cc401523c73a7ea48d433a269a7164059a4812, that the `code, kbd, pre, samp` ruleset was changed from `font-family: monospace, serif;` to `font-family: monospace, monospace;`.

I searched online and found that Eric Meyer tested the monospace, monospace option and it didn't work in his tests. http://meyerweb.com/eric/thoughts/2010/02/12/fixed-monospace-sizing/

For this pull request, I'm going to assume that the change was accidental. If it was done purposefully, can you explain the rationale for the change?

Thanks!