Update integer values for threshold type in network centrality in developer documentation

[jpellman]

Currently:

inputspec.threshold_option: string (int)
threshold options: 0 for probability p_value, 1 for sparsity threshold, any other for threshold value

and

threshold_option : an integer
0 for probability p_value, 1 for sparsity threshold, 2 for actual threshold value, and 3 for no threshold and > fast approach

Looks like 2 for R threshold should be in one or both of these.

https://github.com/FCP-INDI/C-PAC/blob/master/CPAC/network_centrality/resting_state_centrality.py#L831