Update MySQL column type definition

Update 4.4 documentation.

Author: Jérémie Broutier

doctrine/pdo_session_storage.rst

@@ -254,7 +254,7 @@ MySQL
         `sess_id` VARCHAR(128) NOT NULL PRIMARY KEY,
         `sess_data` BLOB NOT NULL,
         `sess_time` INTEGER UNSIGNED NOT NULL,
-        `sess_lifetime` MEDIUMINT NOT NULL
+        `sess_lifetime` INTEGER UNSIGNED NOT NULL
     ) COLLATE utf8mb4_bin, ENGINE = InnoDB;
 
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