fix a spelling pointed out by codespell update (#1563)

EOS/helmholtz/actual_eos.H

@@ -26,7 +26,7 @@ using namespace eos_rp;
 // thermodynamic quantities. of prime interest is the pressure [erg/cm**3],
 // specific thermal energy [erg/gr], the entropy [erg/g/K], along with
 // their derivatives with respect to temperature, density, abar, and zbar.
-// other quantites such the normalized chemical potential eta (plus its
+// other quantities such the normalized chemical potential eta (plus its
 // derivatives), number density of electrons and positron pair (along
 // with their derivatives), adiabatic indices, specific heats, and
 // relativistically correct sound speed are also returned.