Incorrect variable log_slow_query_file_name in KB

https://mariadb.com/kb/en/server-system-variables/#log_slow_query
(the link intentionally points at the previous variable, so it remains correct after the target variable is changed):

log_slow_query_file_name

Description: Name of the slow query log file. Before MariaDB 10.11, was named slow_query_log_file.
Commandline: --log-slow-query-file-name=file_name

In the final version of MDEV-7567, the variable name is log_slow_query_file.

Also needs to be changed where it is referenced in the description of the alias.