docs: fix register_font docstring

[ExploreMaths]

Overview: What does this pull request change?

Fixed the discrepancy between the third directory lookup path described in register_font and the actual code.

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

When I was using register_font, I noticed that the documentation stated:

1. Absolute path.
2. In ``assets/fonts`` folder.
3. In ``font/`` folder.          # Here it says "font"
4. In the same directory.

But the actual code reads:

possible_paths = [
    Path(font_file),
    input_folder / "assets/fonts" / font_file,
    input_folder / "fonts" / font_file,          # Here it says "fonts"
    input_folder / font_file,
]

I found this inconsistency between the two, so I changed the description and documentation from In ``font/`` folder. to In ``fonts/`` folder.

Links to added or changed documentation pages

https://manimce--4880.org.readthedocs.build/en/4880/

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