Replace battery package link with battery_plus

[vbuberen]

Saw that page about using packages has a link to battery package, which is discontinued. Replaced that link with a link to battery_plus, so the page mentions only actively maintained plugins.

Presubmit checklist

• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

[khanhnwin]

Thanks @vbuberen!

[kevmoo]

@khanhnwin @domesticmouse – one of ya'll will likely have to hit the merge button!