Fix section numbers for 'Assignment

[lewisthagichu]

Active Record Queries lesson: fix numbering in Assignment

Because

This contribution resolves numbering inconsistencies in the Active Record Querying's assignment section.

This PR

• Existence of Objects' section number changed from 21 to 20
• Calculations' section number changed from 22 to 21
• Joining Tables' section number changed from 13 to 12
• Find or Build a New Object' section number changed from 19 to 18
• Scopes' section number changed from 15 to 14
• Finding by SQL' section number changed from 20 to 19

Issue

Closes #26465

Additional Information

Pull Request Requirements

• I have thoroughly read and understand The Odin Project Contributing Guide
• The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• If any lesson files are included in this PR, they follow the Layout Style Guide

[linkonsat]

Thanks for doing this. It looks good!