Typo in class name of colorblindness label

Corrected `checkbbx` to `checkbox` in class name of colorblindness label to fix layout bug caused by that.
Update _posts/2013-01-14-alt-text.md

Formatting the list elements correctly.
Merge pull request #74 from nimbupani/patch-1

Just wanted to make this tip more readable
Merge pull request #73 from chrisradford/gh-pages

Fix for layout bug on Checklist page under Testing (second checkbox)
merged from upstream