Fix typo in Using Transforms examples

Match CSharp example with GDScript example

tutorials/3d/using_transforms.rst

@@ -374,7 +374,7 @@ Converting a rotation to quaternion is straightforward.
 
     // Convert basis to quaternion, keep in mind scale is lost
     var a = transform.basis.Quat();
-    var b = transform.basis.Quat();
+    var b = transform2.basis.Quat();
     // Interpolate using spherical-linear interpolation (SLERP).
     var c = a.Slerp(b, 0.5f); // find halfway point between a and b
     // Apply back