Fix heading level

Perhaps they should be in the lower heading level than [*カスタマイズ済みのFirefox・Thunderbirdの展開*](https://github.com/mozilla-japan/enterprise/blob/master/FAQ.md#%E3%82%AB%E3%82%B9%E3%82%BF%E3%83%9E%E3%82%A4%E3%82%BA%E6%B8%88%E3%81%BF%E3%81%AEfirefoxthunderbird%E3%81%AE%E5%B1%95%E9%96%8B).