Replaced <a> with <button> on docs site.
Replaced <a> with <button> on docs site.

Changed element for better semantics and keyboard support under OSX.

gionkunz/chartist-js,chartist-js/chartist,cr3ative/chartist-js,gionkunz/chartist-js,hansmaad/chartist-js,hansmaad/chartist-js,chartist-js/chartist,chartist-js/chartist,cr3ative/chartist-js