FlappyBug: Flap less aggressively

The current flap strength makes the game a lot more difficult than
other flappy games. Decrease the flap strength to make it a little
easier to get higher scores.

Before this change I could only get past 3 or 4 obstacles, now I can
get 15 or 20 in, which seems more on par with other flappy games.

Userland/Games/FlappyBug/Game.h

@@ -65,7 +65,7 @@ private:
 
         void flap()
         {
-            const float flap_strength = 15.0f;
+            const float flap_strength = 10.0f;
             velocity = -flap_strength;
         }