Added language attribute to checklist.html page.

Declaring a language attribute on the html element enables a screen reader to read out the text with correct pronunciation. Not all websites are in english.

Signed-off-by: Sang-Min Yoon <SangMinYoon@Sang-Mins-MacBook-Pro.local>
Merge pull request #230 from smyoon315/gh-pages

Added language attribute to checklist.html page.