docs(ngClass): fix minor typo.

src/ng/directive/ngClass.js

@@ -62,8 +62,8 @@ function classDirective(name, selector) {
  * @name ng.directive:ngClass
  *
  * @description
- * The `ngClass` allows you to set CSS class on HTML element dynamically by databinding an
- * expression that represents all classes to be added.
+ * The `ngClass` allows you to set CSS classes on HTML an element, dynamically, by databinding
+ * an expression that represents all classes to be added.
  *
  * The directive won't add duplicate classes if a particular class was already set.
  *