More accurately describes what a Transform3D is

As pointed out in Issue #8904, a Transform3D is a 3x4 transformation matrix, not a 4x3. While that issue does point out other areas of concern on this specific page, this update is simply to fix that error. The change can be found on line 35.

(cherry picked from commit 9fdfe11)

tutorials/3d/introduction_to_3d.rst

@@ -28,7 +28,7 @@ node for everything 3D.
 
 Node3Ds have a local transform, which is relative to the parent
 node (as long as the parent node is also of **or inherits from** the type
-Node3D). This transform can be accessed as a 4×3
+Node3D). This transform can be accessed as a 3×4
 :ref:`Transform3D <class_Transform3D>`, or as 3 :ref:`Vector3 <class_Vector3>`
 members representing location, Euler rotation (X, Y and Z angles) and
 scale.