Update documentation

[newyork-anthonyng]

No description provided.

[taion]

The docs are referring to the exported constants in

react-transition-group/src/Transition.js

Lines 8 to 11 in df4f6e4

	export const EXITED = 'exited';
	export const ENTERING = 'entering';
	export const ENTERED = 'entered';
	export const EXITING = 'exiting';

[newyork-anthonyng]

@taion I made the updates; thanks for catching that.

[taion]

Thanks!