Single Player → single-player
Summary:
Single-player is proper English, single player and singleplayer are less correct.
(Contrary to multiplayer, which doesn't have a hyphen or space.)
Test Plan: Check for mistakes and omissions.
Reviewers: Gallaecio
Reviewed By: Gallaecio
Subscribers: Freagarach, elexis, O8 JS GUI
Differential Revision: https://code.wildfiregames.com/D2312