Drop extra semicolon in VARIABLE_COMMENT for SYSTEM_VERSIONING_ALTER_HISTORY

MariaDB [test]> select * from information_schema.system_variables where variable_name like 'system_versioning%' \G

*************************** 1. row ***************************

        VARIABLE_NAME: SYSTEM_VERSIONING_ALTER_HISTORY

        SESSION_VALUE: KEEP

         GLOBAL_VALUE: ERROR

  GLOBAL_VALUE_ORIGIN: SQL

        DEFAULT_VALUE: ERROR

       VARIABLE_SCOPE: SESSION

        VARIABLE_TYPE: ENUM

     VARIABLE_COMMENT: Versioning ALTER TABLE mode. ERROR: Fail ALTER with error; KEEP: Keep historical system rows and subject them to ALTER; 

    NUMERIC_MIN_VALUE: NULL

    NUMERIC_MAX_VALUE: NULL

   NUMERIC_BLOCK_SIZE: NULL

      ENUM_VALUE_LIST: ERROR,KEEP

            READ_ONLY: NO

COMMAND_LINE_ARGUMENT: REQUIRED

*************************** 2. row ***************************

        VARIABLE_NAME: SYSTEM_VERSIONING_ASOF

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