Update builtins.md (#4108)

[Preview](https://docs-getdbt-com-git-dbeatty-builtins-context-variable-dbt-labs.vercel.app/reference/dbt-jinja-functions/builtins)

## What are you changing in this pull request and why?
`builtins` is a variable rather than a function. But it does contain
references to the builtin dbt context methods:
- [ref](https://docs.getdbt.com/reference/dbt-jinja-functions/ref)
- [source](https://docs.getdbt.com/reference/dbt-jinja-functions/source)
- [config](https://docs.getdbt.com/reference/dbt-jinja-functions/config)

## Checklist
- [x] Review the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md)
and [About
versioning](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#adding-a-new-version)
so my content adheres to these guidelines.

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

@@ -1,8 +1,8 @@
 ---
-title: "About builtins Jinja function"
+title: "About builtins Jinja variable"
 sidebar_label: "builtins"
 id: "builtins"
-description: "Read this guide to understand the builtins Jinja function in dbt."
+description: "Read this guide to understand the builtins Jinja variable in dbt."
 ---