Update map_grid header

prep_regions/map_grid.f90

@@ -32,7 +32,7 @@
 !!             are >99% water are given negative numbers while boxes that are land are
 !!             given positive numbers.
 !!
-!! Options: 
+!! Options: (defined in SETTINGS_regions)
 !!             useflux         -  (true/false) weight surface influence by annual fluxes
 !!             uselanduse      -  (true/false) avoid gridcells with > 99% water
 !!             nmap            -  (integer) number of resolution steps to use