fix(user-manual): remove self-referential doc link in userpreferences

AI-Assisted-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Anna Larch <anna@nextcloud.com>

Author: miaulalala

user_manual/userpreferences.rst

@@ -31,7 +31,7 @@ include the following:
 * List of your Group memberships
 * Change your password
 * :doc:`user_2fa`
-* :doc:`userpreferences`
+* User preferences
 * Choose the language for your Nextcloud interface
 * Choose your preferred first day of the week
 * Links to desktop and mobile apps