Fix typo
Merge branch 'master' of github.com:w3c/wai-tutorials

# By Anna Belle Leiserson (3) and Eric Eggert (1)
* 'master' of github.com:w3c/wai-tutorials:
  Update index.html.erb.md
  Minor copy edit
  Minor changes to phrasing
  Minor changes to phrasing
Punctuation

Fixes #144