fix doc typo
Merge pull request #4539 from denisnazarov/patch-3

fix doc typo
ES6 ember-handlebars & ember-handlebars-compiler: 1.
ES6 ember-handlebars & ember-handlebars-compiler: 2.
ES6 ember-handlebars & ember-handlebars-compiler: 3.
ES6 ember-handlebars & ember-handlebars-compiler: 4.