docs: update r2dbc url for MariaDB

[wan92hen]

根据 halo-dev/halo#5534 (comment) 中提到的，临时将文档中 MariaDB 的数据库连接地址修改为使用 mysql 驱动。

None

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[ruibaby]

@wan92hen 需要同时修改一下 https://github.com/halo-dev/docs/blob/main/docs/getting-started/install/slots/docker-args.md

[ruibaby]

/lgtm