Tightens Underdog language in the README.md

The previous language implied that an impostor had to die to trigger the
underdog. This makes it clear how things work in 1x or 3x impostor
games.

README.md

@@ -680,7 +680,7 @@ The Undertaker is an Impostor that can drag and drop bodies.
 ### **Team: Impostors**
 
 The Underdog is an Impostor with a prolonged kill cooldown.\
-Once their partener dies they will have their kill cooldown shortened.
+When they are the only remaining Impostor, they will have their kill cooldown shortened.
 
 ### Game Options
 | Name | Description | Type | Default |