Fixes the APC full_charge mapping helper (#76650)

## About The Pull Request
Currently the full_charge APC helper just sets the cell's charge to 100
flat, instead of 100%. This PR fixes that by setting the cell's charge
to whatever the cell's max charge is.

This is also my first PR. I was hoping to add a whiteship or something
instead but a fix is good too.

## Why It's Good For The Game
A mapping helper for giving APCs a full charge should actually fully
charge the APCs, instead of giving them only ~5% charge.

## Changelog
🆑
fix: Fully charged APCs will now actually be fully charged, instead of
nearly empty.
/🆑

code/modules/power/apc/apc_main.dm

@@ -705,7 +705,7 @@
 
 /// Used for full_charge apc helper, which sets apc charge to 100%.
 /obj/machinery/power/apc/proc/set_full_charge()
-	cell.charge = 100
+	cell.charge = cell.maxcharge
 
 /*Power module, used for APC construction*/
 /obj/item/electronics/apc