doc: Fix markup of a clode block in Texture Healing.md

[tadd]

It looks like that markup of a code block in Texure Healing.md is broken below ".. are defined, such as:," like "**lookup**."

[idan]

I think this looks ok, but not really knowing the "language" that drives these internals I have a hard time checking if the updated version is correct.

@heathercran @rileycran does this look okay to you?