docs(ngClass): fix grammar

Closes angular#4061

Author: jdavisp3

src/ng/directive/ngClass.js

@@ -215,10 +215,10 @@ var ngClassDirective = classDirective('', true);
  *
  * @description
  * The `ngClassOdd` and `ngClassEven` directives work exactly as
- * {@link ng.directive:ngClass ngClass}, except it works in
- * conjunction with `ngRepeat` and takes affect only on odd (even) rows.
+ * {@link ng.directive:ngClass ngClass}, except they work in
+ * conjunction with `ngRepeat` and take effect only on odd (even) rows.
  *
- * This directive can be applied only within a scope of an
+ * This directive can be applied only within the scope of an
  * {@link ng.directive:ngRepeat ngRepeat}.
  *
  * @element ANY
@@ -263,10 +263,10 @@ var ngClassOddDirective = classDirective('Odd', 0);
  *
  * @description
  * The `ngClassOdd` and `ngClassEven` directives work exactly as
- * {@link ng.directive:ngClass ngClass}, except it works in
- * conjunction with `ngRepeat` and takes affect only on odd (even) rows.
+ * {@link ng.directive:ngClass ngClass}, except they work in
+ * conjunction with `ngRepeat` and take effect only on odd (even) rows.
  *
- * This directive can be applied only within a scope of an
+ * This directive can be applied only within the scope of an
  * {@link ng.directive:ngRepeat ngRepeat}.
  *
  * @element ANY