Fix documentation on split_grouping_derived and split_materialized

MariaDB 10.3 supposedly added a new optimization related to derived tables:

New optimizer switch setting, split_grouping_derived=on (see description)

https://mariadb.com/kb/en/library/changes-improvements-in-mariadb-103/#optimizerperformance

split_grouping_derived=on MariaDB 10.3.1

https://mariadb.com/kb/en/library/optimizer-switch/#available-flags

However, at some point, the split_grouping_derived optimizer_switch was removed, and it was replaced with split_materialized. The documentation was never updated.