Rename update function `dot_position` to `update_label` in `.add_updater` example

[irvanalhaq9]

Overview: What does this pull request change?

Renamed update function dot_position to update_label in .add_updater example.

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

• The previous function name dot_position was misleading because it suggested updating the dot's position.
• The function actually updates the label's value and the label's position.
• Renaming it to update_label improves clarity and better reflects its purpose.

Links to added or changed documentation pages

https://manimce--4196.org.readthedocs.build/en/4196/reference/manim.mobject.mobject.Mobject.html#manim.mobject.mobject.Mobject.add_updater

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