Update entity requirement in Button Card docs

[dale3h]

The Lovelace Button Card fails to render in the latest version (1.0.0b0) if entity is not specified.

This might also need to be added to release notes as a breaking change.

Based on the card editor UI this appears to now be required, but I couldn't find any references to it in PRs, issues, nor RC release notes.

Proposed change

Update docs for Lovelace Button Card to show new entity requirement.

Type of change

• Spelling, grammar or other readability improvements (current branch).
• Adjusted missing or incorrect information in the current documentation (current branch).
• Added documentation for a new integration I'm adding to Home Assistant (next branch).
  • I've opened up a PR to add logo's and icons in Brands repository.
• Added documentation for a new feature I'm adding to Home Assistant (next branch).
• Removed stale or deprecated documentation.
• Adjusted missing or incorrect information in the upcoming documentation (next branch).

Additional information

Example:

type: button
name: Netflix
icon: 'mdi:netflix'
icon_height: 64px
tap_action:
  action: call-service
  service: media_player.select_source
  service_data:
    entity_id: media_player.living_room_fire_tv
    source: com.netflix.ninja
hold_action:
  action: call-service
  service: androidtv.adb_command
  service_data:
    entity_id: media_player.living_room_fire_tv
    command: >-
      sendevent /dev/input/event5 4 4 786979 && sendevent /dev/input/event5 1
      172 1 && sendevent /dev/input/event5 0 0 0 && sendevent /dev/input/event5
      4 4 786979 && sendevent /dev/input/event5 1 172 0 && sendevent
      /dev/input/event5 0 0 0

• Link to parent pull request in the codebase:
• Link to parent pull request in the Brands repository:
• This PR fixes or closes issue:

Checklist

• This PR uses the correct branch, based on one of the following:
  • I made a change to the existing documentation and used the current branch.
  • I made a change that is related to an upcoming version of Home Assistant and used the next branch.
• The documentation follows the Home Assistant documentation standards.

[SeanPM5]

This was just a bug, a fix was merged a few minutes ago here: home-assistant/frontend#7909 and should be in the next beta.

[frenck]

Closing as this is a bug.