fix typo (#1316)

Source/missiles.h

@@ -56,7 +56,7 @@ typedef struct MissileStruct {
 	Sint32 _mityoff; // How far the missile has travelled in its lifespan along the Y-axis. mix/miy/mxoff/myoff get updated every game tick based on this
 	Sint32 _mimfnum; // The direction of the missile (direction enum)
 	Sint32 _mispllvl;
-	bool _miDelFlag; // Indicate weather the missile should be deleted
+	bool _miDelFlag; // Indicate whether the missile should be deleted
 	Uint8 _miAnimType;
 	Sint32 _miAnimFlags;
 	Uint8 *_miAnimData;