[docs] Fix formatting in functions/json.rst

[steveburnett]

Description

Fix formatting in functions/json.rst to remove vertical gray lines in text.

Motivation and Context

Fix display text in the documentation.

Impact

Documentation.

Test Plan

Local doc builds.

Screenshots before:

Screenshots after:

Contributor checklist

• Please make sure your submission complies with our development, formatting, commit message, and attribution guidelines.
• PR description addresses the issue accurately and concisely. If the change is non-trivial, a GitHub Issue is referenced.
• Documented new properties (with its default value), SQL syntax, functions, or other functionality.
• If release notes are required, they follow the release notes guidelines.
• Adequate tests were added if applicable.
• CI passed.

Release Notes

== NO RELEASE NOTE ==

[imjalpreet]

LGTM, just a small nit.