fix: correcting jinja graph exposure property name from "source" to "…

…exposure" (#4116)

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

This is a small fix to correct the docs on the DBT jinja graph
components. The dbt graph element for exposures are currently
incorrectly documented as having a unique_id namespaced by `source`,
when in fact they are namespaced by `exposure`.

This PR makes that small correction.

## Checklist
No applicable checklist items.

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

@@ -99,7 +99,7 @@ representations of those nodes. A simplified example might look like:
   },
   "exposures": {
     "exposure.my_project.traffic_dashboard": {
-      "unique_id": "source.my_project.traffic_dashboard",
+      "unique_id": "exposure.my_project.traffic_dashboard",
       "type": "dashboard",
       "maturity": "high",
       "path": "models/path/to/schema.yml",