Fix the Angstrom.Buffered.state_to_option docstring

[anmonteiro]

The docstring claimed to return Some (bs, v) when in reality it returns Some v.

Refs:

angstrom/lib/angstrom.ml

Lines 118 to 120 in 3a3e245

	let state_to_option = function
	| Done(_, v) -> Some v
	| _ -> None

[seliopou]

Thanks!