docs(ngSrc): srcset used instead of src for img attribute
docs(ngSrc): srcset used instead of src for img attribute

In `ngSrc` documentation `srcset` is used instead of `src` as `img` element attribute in the example.

Closes #7951

ggershoni/angular.js,ggershoni/angular.js,ggershoni/angular.js,ggershoni/angular.js