events/event-basics.md: inserted missing semicolon into example code

Closes gh-541
events/basics: fix create button DOM element example

Closes gh-587
using-jquery-core/selecting-elements.md: ensured consistent teg representation in content.

Closes gh-564

Formatted <select> in paragraph for :checked selector
javascript-101/loops: add for...in loop

Closes gh-588
Fixes gh-321