Fix parameter name in DirectorySync#list_users documentation

[masamichhhhi]

Description

The parameter name in the documentation was incorrectly listed as user when it should be group according to the folloiwing API reference documentation. https://workos.com/docs/reference/directory-sync/directory-user/list

Documentation

Does this require changes to the WorkOS Docs? E.g. the API Reference or code snippets need updates.

[ ] Yes

If yes, link a related docs PR and add a docs maintainer as a reviewer. Their approval is required.