Fix menu display for the Jinja return function (#4606)

resolves: #4605

## What are you changing in this pull request and why?
This was the only thing I could find that said `sidebar_variable`.
Everything else had `sidebar_label` instead.

## Checklist
- [x] Review the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md)
so my content adheres to these guidelines.

website/docs/reference/dbt-jinja-functions/return.md

@@ -1,6 +1,6 @@
 ---
 title: "About return function"
-sidebar_variable: "return"
+sidebar_label: "return"
 id: "return"
 description: "Read this guide to understand the return Jinja function in dbt."
 ---