Removed mode: mono

[sanchosk]

Description:

Seems like there is error in docs - mono seems not to be valid option in mode:, only left/right/stereo are allowed.

Checklist:

• [ ] I am merging into next because this is new documentation that has a matching pull-request in esphome as linked above.
  or

• [X] I am merging into current because this is a fix, change and/or adjustment in the current documentation and is not for a new component or feature.

• [ ] Link added in /index.rst when creating new documents for new components or cookbook.