[Doc] Incorrect target listing for sys.db classes

Classes such as [sys.db.Mysql](http://api.haxe.org/sys/db/Mysql.html) and [sys.db.Sqlite](http://api.haxe.org/sys/db/Sqlite.html) are said to target Python in the documentation.

However, there is no code for this target, resulting in the default execution error : `Not implemented for this platform`