minor edit to h-h text in modeling api doc

Author: Alexander Ororbia

docs/modeling/neurons.md

@@ -234,7 +234,7 @@ fast spiking (FS), low-threshold spiking (LTS), and resonator (RZ) neurons.
 This cell models dynamics over voltage `v` and three channels/gates (related to 
 potassium and sodium activation/inactivation). This sophisticated cell system is, 
 as a result,  a set of four coupled differential equations and is driven by an appropriately  configured set of biophysical constants/coefficients (default values of which have  been set according to relevant source work). 
-(Note that this cell supports either Euler or midpoint method / RK-2 integration.)
+(Note that this cell supports Euler, midpoint / RK-2 integration, or RK-4 integration.)
 
 ```{eval-rst}
 .. autoclass:: ngclearn.components.HodgkinHuxleyCell