Fixing a bug with label color in graph not applying.

[barollet]

Overview: What does this pull request change?

Fixes a bug with label color in Graph Mobject being always white (the default color for MathTex) because of wrong argument name.

Motivation and Explanation: Why and how do your changes improve the library?

I like to have good argument names so we can actually use it haha.

Links to added or changed documentation pages

Further Information and Comments

Actually I thought my last PR with 4 lines diff would be the smallest I do, but this one is actually smaller. Maybe I'll manage to make a 1 line PR at some point.

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested

[barollet]

TestScene.mp4

Better with colors, compare with actual documentation https://docs.manim.community/en/stable/reference/manim.mobject.graph.Graph.html#labeledmodifiedgraph