Use u"g" for grams in generate-table-entries.jl

UnitfulAstro.jl doesn't import grams from Unitful, so the symbol by
default cannot be used. But it can be looked up through u_str/u"" macro

docs/generate-table-entries.jl

@@ -15,17 +15,17 @@ table = (("Erg", :erg, :J),
          ("Solar irradiance", :Ssun, u"erg/s/cm^2"),
          ("Solar luminosity", :Lsun, u"erg/s"),
          ("G × Solar mass", :GMsun, u"m^3/s^2"),
-         ("Solar mass", :Msun, :g),
+         ("Solar mass", :Msun, u"g"),
          ("Earth radius (equatorial)", :Rearth_e, :cm),
          ("Earth radius (polar)", :Rearth_p, :cm),
          ("Earth radius", :Rearth, :cm),
          ("G × Earth mass", :GMearth, u"m^3/s^2"),
-         ("Earth mass", :Mearth, :g),
+         ("Earth mass", :Mearth, u"g"),
          ("Jupiter radius (equatorial)", :Rjup_e, :cm),
          ("Jupiter radius (polar)", :Rjup_p, :cm),
          ("Jupiter radius", :Rjup, :cm),
          ("G × Jupiter mass", :GMjup, u"m^3/s^2"),
-         ("Jupiter mass", :Mjup, :g),
+         ("Jupiter mass", :Mjup, u"g"),
          ("Fifty-one ergs", :foe, :erg),
          ("Solar flux unit", :SFU, :kJy),
          ("Total electron content unit", :TECU, u"cm^-2"))