update references to Bootstrap

Bootstrap is no longer organizationally affiliated with Twitter
Bootstrap's website URLs have changed
Also fixes some typos I came across
Merge branch 'patch-1' of github.com:cvrebert/a11yproject.com into cvrebert-patch-1

* 'patch-1' of github.com:cvrebert/a11yproject.com:
  update references to Bootstrap
video additions
adding WebAIM to blog section as well
adding Chrome accessibility tools
adding book recommendation
adding link to introduction to screenreader testing. It may not belong under this heading, but provides really useful general tips. For beginners, a Link-and-run straight to screen readers may be confusing?
moving HTML5 accessibility down the list, on it's own it isn't super useful when looking up tips on accessible HTML? Next, updating the link for recommended ARIA usage (old link was redirecting), and adding a new W3C resource on HTML specifically