Documentation fix: dx vs. x0

Summary:
> What existing problem does the pull request solve?

Purely a documentation/example error. Was misled/wasted time debugging because of this issue.

Description:
`dx`/`dy` are zero in onPanResponderGrant, not `x0`/`y0`. `x0`/`y0` will be the coordinates relative to the granted element, which are rarely zero.
Closes https://github.com/facebook/react-native/pull/9984

Differential Revision: D4001603

Pulled By: hramos

fbshipit-source-id: 32580795f62394585ccfb8f9a2ae65b0b72add69