Fix the key-name now that we learned the documentation is out of date.

phabricator sez:

    The configuration option 'maniphest.custom-fields' has been removed. You may delete it at your convenience.

    Maniphest fields are now defined in `maniphest.custom-field-definitions`. Existing definitions have been migrated.

    This configuration value is defined in this configuration source: File 'khan'.

Test Plan:
Fingers crossed.