Increases the amount of mk2/m39 mags in preps (#7303)

# About the pull request
When I made #7153 I decreased the starting mk2 and m39 mags too much, so
i'm increasing them again
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# Explain why it's good for the game
I want more mk2 mags in prep vendor so that I don't need to loot
multiple vendors for ammo.
# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>

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# Changelog
:cl:
balance: Increased the starting amount of mk2 and m39 mags in prep
vendors
/:cl:

code/game/machinery/vending/vendor_types/squad_prep/squad_prep.dm

@@ -27,8 +27,8 @@
 		list("Box of Buckshot Shells (12g)", floor(scale * 5), /obj/item/ammo_magazine/shotgun/buckshot, VENDOR_ITEM_REGULAR),
 		list("Box of Shotgun Slugs (12g)", floor(scale * 5), /obj/item/ammo_magazine/shotgun/slugs, VENDOR_ITEM_REGULAR),
 		list("M4RA Magazine (10x24mm)", floor(scale * 15), /obj/item/ammo_magazine/rifle/m4ra, VENDOR_ITEM_REGULAR),
-		list("M39 HV Magazine (10x20mm)", floor(scale * 10), /obj/item/ammo_magazine/smg/m39, VENDOR_ITEM_REGULAR),
-		list("M41A Magazine (10x24mm)", floor(scale * 10), /obj/item/ammo_magazine/rifle, VENDOR_ITEM_REGULAR),
+		list("M39 HV Magazine (10x20mm)", floor(scale * 15), /obj/item/ammo_magazine/smg/m39, VENDOR_ITEM_REGULAR),
+		list("M41A Magazine (10x24mm)", floor(scale * 15), /obj/item/ammo_magazine/rifle, VENDOR_ITEM_REGULAR),
 
 		list("SIDEARMS", -1, null, null),
 		list("88 Mod 4 Combat Pistol", floor(scale * 10), /obj/item/weapon/gun/pistol/mod88, VENDOR_ITEM_REGULAR),