[docs] Fix formatting in sql/show-create-schema.rst

[steveburnett]

Description

Fix formatting in sql/show-create-schema.rst.

Motivation and Context

Reduce the number of warnings in doc builds, local and otherwise.

Impact

Doc build time.

Test Plan

Local doc builds.

BEFORE:

/Users/steveburnett/Documents/GitHub/presto/presto-docs/src/main/sphinx/sql/show-create-schema.rst:1: WARNING: Title overline too short.

=================
SHOW CREATE SCHEMA
================= [docutils]

AFTER:
The warning shown above does not appear in a local doc build.

Contributor checklist

• Please make sure your submission complies with our contributing guide, in particular code style and commit standards.
• 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 ==

[hantangwangd]

Got it. Thanks for the fix.