Assigning to COLOR in fragment() gives error "Constants cannot be modified."

**Your Godot version:** 4.7 RC2

**Issue description:** The intro documentation page on shaders has example code that would produce the error "Constants cannot be modified.". COLOR is only an input variable to fragments and cannot be output. The code is incomplete anyway but can be misleading.

**URL to the documentation page:** https://docs.godotengine.org/en/stable/tutorials/shaders/introduction_to_shaders.html