Improve docs for convertColors

[JohnAlbin]

The docs for most of the plugins are… scant.

I was writing a custom plugin when I realized that the convertColors plugin already did what I wanted, but the currentColor param was completely undocumented.

Here's a PR to fix that part of the docs.

[TrySound]

Please run "npm run fix"

[SethFalco]

Thanks for opening the PR and proposing we document this behavior. This has been resolved through publishing documentation svgo.dev.

You can read more about convertColor and the currentColor param here:
https://svgo.dev/docs/plugins/convert-colors/#parameters

Contributions to improve our documentation are welcome! 👍🏽