Fix the configuration help string to use the correct name for SPHENIX

configure.ac

@@ -1618,7 +1618,7 @@ esac
 # Hydro scheme.
 AC_ARG_WITH([hydro],
    [AS_HELP_STRING([--with-hydro=<scheme>],
-      [Hydro dynamics to use @<:@gadget2, minimal, pressure-entropy, pressure-energy, pressure-energy-monaghan, default, gizmo-mfv, gizmo-mfm, shadowfax, planetary, anarchy-du, anarchy-pu default: gadget2@:>@]
+      [Hydro dynamics to use @<:@gadget2, minimal, pressure-entropy, pressure-energy, pressure-energy-monaghan, default, gizmo-mfv, gizmo-mfm, shadowfax, planetary, sphenix, anarchy-pu default: gadget2@:>@]
    )],
    [with_hydro="$withval"],
    [with_hydro="gadget2"]