Wrong variable names for temporary space usage in documentation

Two system variables limiting temporary space were introduced in the scope of MDEV-9101.
In the initial implementation they were named max_tmp_space_usage and max_total_tmp_space_usage, but in the final version they were renamed to max_tmp_session_space_usage and max_tmp_total_space_usage; however in the KB they are still documented under the original names (https://mariadb.com/kb/en/server-system-variables/#max_tmp_space_usage, https://mariadb.com/kb/en/server-system-variables/#max_total_tmp_space_usage)