changed ACT to opt

Co-authored-by: Eric Sorenson <ahpook@github.com>

Author: Drwhomust

topics/undertale/index.md

@@ -14,5 +14,5 @@ Undertale is a video game by Toby Fox. Released in 2015, The game is about
 the player controlling a small child who fell into a pit near mount ebott.
 And separated by a magical barrier created by the humans in the past.
 You have to go though a journey back to the surface while coming across
-monsters. You can ACT to spare monsters or kill them. There are many endings
+monsters. You can opt to spare monsters or kill them. There are many endings
 depending on how you play the game