Fix missing semicolon in C# code sample in Exporting tutorial (#3778)

Player.cs code (c#) won't compile because of a missing simicolon.

getting_started/step_by_step/exporting.rst

@@ -170,7 +170,7 @@ changed:
             // Move towards the target and stop when close.
             if (Position.DistanceTo(_target) > 10)
             {
-                velocity = _target - Position
+                velocity = _target - Position;
             }
 
             // Remove keyboard controls.