Update lyrics.rst

[swishyswoosh]

looks like this backend was replaced with musixmatch

Description

Fixes #X.

(...)

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[jackwilsdon]

The defaults for 1.6.0 (the latest release) are google, musixmatch, genius and tekstowo:

beets/beetsplug/lyrics.py

Line 688 in 1937180

SOURCES = ['google', 'musixmatch', 'genius', 'tekstowo']

This was correctly documented at the time: https://github.com/beetbox/beets/blob/v1.6.0/docs/plugins/lyrics.rst#configuration

Since 1.6.0, the new lrclib source has been added (in #4989) and musixmatch has been removed from the default list (in #4616). These changes aren't yet in a release but are present on master, so the documentation is correct.

You can also see the difference on the hosted documentation:

• 1.6.0: https://beets.readthedocs.io/en/v1.6.0/plugins/lyrics.html#configuration
• master: https://beets.readthedocs.io/en/latest/plugins/lyrics.html#configuration