Fixes the 88 Mod 4's holster sprite (#5785)

# About the pull request

**no actual sprite changes, I just renamed a couple**

Fixes the 88 Mod 4 pistol not having a holster sprite, and showing as a
bunch of orange question marks.
This was happening because #5729 changed its `icon_state` from `"88m4"`
to `"_88m4"` in order to make it actually work with CSS, but the sprite
in `icons/obj/items/clothing/belts.dmi` wasn't renamed with it.

I also changed the `item_state` to be the same and renamed the inhand
sprites, just to reduce any future confusion.

# Explain why it's good for the game

Fixes the gun not being a gun.

# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>

**Before:**

![before](https://github.com/cmss13-devs/cmss13/assets/57483089/44ca7d17-440a-4ae2-8fbe-1df38a80e6f3)

**After:**

![after](https://github.com/cmss13-devs/cmss13/assets/57483089/ad27fe53-4d36-4101-9122-135dad0bf620)

</details>


# Changelog
:cl:
fix: Fixed the 88 Mod 4 pistol's holster sprite.
/:cl:

code/modules/projectiles/guns/pistols.dm

@@ -587,7 +587,7 @@
 	desc = "Standard issue USCM firearm. Also found in the hands of Weyland-Yutani PMC teams. Fires 9mm armor shredding rounds and is capable of 3-round burst."
 	icon = 'icons/obj/items/weapons/guns/guns_by_faction/uscm.dmi'
 	icon_state = "_88m4" // to comply with css standards
-	item_state = "88m4"
+	item_state = "_88m4"
 	fire_sound = "88m4"
 	firesound_volume = 20
 	reload_sound = 'sound/weapons/gun_88m4_reload.ogg'