FlappyBug: Standardize on "high score" rather than "highscore"

"High score" should be two words, and this matches other games in the
system.

Userland/Games/FlappyBug/Game.h

@@ -143,7 +143,7 @@ private:
     Obstacle m_obstacle;
     Cloud m_cloud;
     bool m_active;
-    Optional<float> m_highscore {};
+    Optional<float> m_high_score {};
     float m_last_score {};
     float m_difficulty {};
     float m_restart_cooldown {};