docs(guide/directive): fix indentation in example code

Closes #4241
docs(angular.bind): clarify that bind is partial application

The `angular.bind` function reflects the definition of "partial application", which
reduces a function's arity rather than transforming a function with n args into a
chain of n functions, each having a single arg.

curry : f(x,y,z) -> f(x)(y)(z)
partial application : f(x,y,z) -> f(x)(y,z)

Closes #4239