docs: replace MySQL with MySQLi

Author: kenjis

user_guide_src/source/database/metadata.rst

@@ -105,7 +105,7 @@ database:
 -  name - column name
 -  type - the type of the column
 -  max_length - maximum length of the column
--  primary_key - integer ``1`` if the column is a primary key (all integer ``1``, even if there are multiple primary keys), otherwise integer ``0`` (This field is currently only available for MySQL and SQLite3)
+-  primary_key - integer ``1`` if the column is a primary key (all integer ``1``, even if there are multiple primary keys), otherwise integer ``0`` (This field is currently only available for MySQLi and SQLite3)
 -  nullable - boolean ``true`` if the column is nullable, otherwise boolean ``false``
 -  default - the default value