Update meta config (#3954)

[Preview](https://deploy-preview-3954--docs-getdbt-com.netlify.app/reference/resource-configs/meta)

## What are you changing in this pull request and why?

This PR is related to
#3710. It handles the
"update examples" part but doesn't handle the "add more explanation"
part.

It fixes two things in particular:
1. Aligns the example for `sources` with the recommended usage (that is
most likely to behave correctly)
1. Removes part of the example for `macros` that raises a Parsing Error

## 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/resource-configs/meta.md

@@ -59,11 +59,13 @@ version: 2
 
 sources:
   - name: model_name
-    meta: {<dictionary>}
+    config:
+      meta: {<dictionary>}
 
     tables:
       - name: table_name
-        meta: {<dictionary>}
+        config:
+          meta: {<dictionary>}
 
         columns:
           - name: column_name
@@ -152,7 +154,6 @@ macros:
 
     arguments:
       - name: argument_name
-        meta: {<dictionary>}
 
 ```