Add explicit leading dot in README link

[philrz]

During my research into an alternate hosting platform for the docs site, I bumped into yet another flavor of broken link in the ZJSON format doc, tied to the text "a super-structured data model". Following the link from the page in its original GitHub home at https://github.com/brimdata/zed/blob/main/docs/formats/zjson.md, it goes to the right place. But if you follow the same link on the same page on the published docs site at https://zed.brimdata.io/docs/next/formats/zjson/, it goes to the wrong place. With the fix in this PR, the page in the test site at https://62fc192ae0abad15aea124c1--spiffy-gnome-8f2834.netlify.app/docs/next/formats/zjson/ goes to the correct place again.