Fixes disguised morph movespeed bug (tgstation#72791)

## About The Pull Request

Morph is described as this in the antag panel: "While morphed, you move
faster, but are unable to attack creatures or eat anything." This is how
it has worked as long as I have played, but at somepoint in the past
this was broken. I was able to track down the source of this issue
easier with admin powers, finally back to squash my own bug report.

## Why It's Good For The Game

Fixes tgstation#61693

## Changelog
:cl:
fix: Morphs now properly move faster when disguised rather than slowing
down.
/:cl:

code/modules/movespeed/modifiers/mobs.dm

@@ -127,7 +127,7 @@
 	multiplicative_slowdown = 5
 
 /datum/movespeed_modifier/morph_disguised
-	multiplicative_slowdown = 1
+	multiplicative_slowdown = -1
 
 /datum/movespeed_modifier/auto_wash
 	multiplicative_slowdown = 3