Update session.rst - SQL statement correction.

The wrong Sql statement:
CREATE TABLE `sessions` (
      `id` varchar(255) NOT NULL DEFAULT '',
      `data` VARBINARY, -- or BYTEA for PostgreSQL
      `expires` int(11) DEFAULT NULL,
      PRIMARY KEY (`id`)
    );

The data type for 'data' field is wrong, and it's not working. Bellow is the right Sql statement
 CREATE TABLE `sessions` (
      `id` varchar(255) NOT NULL DEFAULT '',
      `data` BLOB, -- or BYTEA for PostgreSQL
      `expires` int(11) DEFAULT NULL,
      PRIMARY KEY (`id`)
    );