Fix code block in docs for Using custom OneToOneFields

[bheesham]

Description

Fixes a typo in the documentation.

Related Issue

#1010

Motivation and Context

It fixes some formatting in the generated docs.

How Has This Been Tested?

I viewed the file on GitHub, currently available here: https://github.com/bheesham/django-simple-history/blob/fix-code-block-documentation/docs/common_issues.rst#using-custom-onetoonefields

Screenshots (if appropriate):

See bug report for screenshot of what it was like before: #1010

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have run the pre-commit run command to format and lint.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes. -- None required.
• I have added my name and/or github handle to AUTHORS.rst
• I have added my change to CHANGES.rst -- there was already a line in there for fixing typos in the docs.
• All new and existing tests passed. -- just documentation change so I'm (probably) not breaking anything, some tests failed but that's because I don't have certain things installed.