add main gun rate upgrade to ship pool

vacuum_gambit.p8

@@ -736,8 +736,8 @@ function gun_base:ammo_upgrade_opt()
  before you
   run out.
 
-   is: ]]..tostr(a)..[[
-  add: ]]..tostr(x)..[[
+   is: ]]..tostr(a)..[[ 
+  add: ]]..tostr(x)..[[ 
 ----------
 total: ]]..tostr(a+x),
   action=function()
@@ -1200,7 +1200,9 @@ from further
   action=function ()
    self.magnet += 2
   end,
- }}
+ },
+ self.main_gun.rate_upgrade_opt(),
+}
 
  if cdr > 0 then
   add(ret, {