fix(typo): correct 'third' to 'fourth' in quad.dart (#326)

Author: nadaabdelkhalik

lib/src/vector_math/quad.dart

@@ -20,7 +20,7 @@ class Quad {
   /// The third point of the quad.
   Vector3 get point2 => _point2;
 
-  /// The third point of the quad.
+  /// The fourth point of the quad.
   Vector3 get point3 => _point3;
 
   /// Create a new, uninitialized quad.