Fix typo in EmbedButtonEditorAccessCheck::access() docbloc

The access requirement key is _embed_button_editor_access, not _embed_button_filter_access.
embed.services.yml:

services:
  access_check.embed_button_editor:
    class: Drupal\embed\Access\EmbedButtonEditorAccessCheck
    tags:
      - { name: access_check, applies_to: _embed_button_editor_access }