[x3d-public] Fix one sentence about texturing in X3Dv4

[Michalis Kamburelis]

I just noticed one sentence that needs to be updated in X3Dv4, because
right now -- after all the edits -- it's obviously no longer true.

12.4.2 Appearance,
https://www.web3d.org/specifications/X3Dv4Draft/ISO-IEC19775-1v4-CD/Part01/components/shape.html#Appearance
sentence:

"""
f the texture node is NULL or the texture field is unspecified, the
object that references this Appearance is not textured.
"""

->

"""
The texture node, if it is provided and not NULL, specifies the
texture that plays the role of main texture (diffuse, base, or
emissive texture -- depending on the material type). The exact
treatment of this texture is described in "12.2.5 Coexistence of
textures specified in material nodes with the "Appearance.texture"
field".
"""

Don, can I put on you updating this part of the specification /
putting this in Mantis? Sorry I didn't catch this sentence earlier.

Regards,
Michalis