Add clarity surrounding metadata-* attribute for HTML Mux Player (#1327)

This PR adds a small line about how to reformat Mux Data dimensions such
that they work with the HTML Mux Player `metadata-*` attribute.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk documentation-only change; no runtime code paths are
modified.
> 
> **Overview**
> Clarifies the `metadata-*` attribute docs in
`packages/mux-player/REFERENCE.md` by noting that Mux Data dimension
names must be provided in kebab-case (e.g., `metadata-player-name` vs
`metadata-player_name`) when using `metadata-*` attributes.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
78dcc408f82bac1f343b4375408eb3d5a007cb5e. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->