doc: fix link to JSON formatter
<p>
When using the <a href="/docs/formatters#text"><code>text</code></a> or <a href="/docs/formatters#stylish"><code>stylish</code></a> formatters, related information will
- appear as indented lines. The <a href="/docs/formatters@json"><code>json</code></a> formatter adds a new field
+ appear as indented lines. The <a href="/docs/formatters#json"><code>json</code></a> formatter adds a new field
<code>related</code> to problems, containing position information as well as the
message. Editors that use gopls will also display the related
information.
