Update api.md
Update api.md

This patch improves on api.md so that the raw md is human-readable
and the rendering is still good.

The next step would be linting markdown so that it's up-to-date and neat.

References #14.
a1ph/puppeteer,brutalcrozt/puppeteer,ianchn/puppeteer,puppeteer/puppeteer,puppeteer/puppeteer,puppeteer/puppeteer,michaelfward/puppeteer,elisherer/puppeteer,elisherer/puppeteer,brutalcrozt/puppeteer,GoogleChrome/puppeteer,michaelfward/puppeteer,ianchn/puppeteer,a1ph/puppeteer,GoogleChrome/puppeteer,puppeteer/puppeteer,GoogleChrome/puppeteer