Update tutorial-document-your-models.md (#4121)

Add customers.customer_id from the previous step

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Quickstart for dbt Cloud and BigQuery
Step 12 Add tests to your models
https://docs.getdbt.com/quickstarts/bigquery?step=12
Step 13 Document your models
https://docs.getdbt.com/quickstarts/bigquery?step=13

I found that the schema.yml in step 13 is not consistent with step 12.

There is column:customer_id in step 12 but not step 13. I would like it
to be added to step 13 as well.

```
      - name: customer_id
        tests:
          - not_null
          - relationships:
              to: ref('stg_customers')
              field: customer_id
``` 


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Co-authored-by: Leona B. Campbell <[email protected]>

website/snippets/tutorial-document-your-models.md

@@ -40,7 +40,12 @@ Adding [documentation](/docs/collaborate/documentation) to your project allows y
             tests:
               - accepted_values:
                   values: ['placed', 'shipped', 'completed', 'return_pending', 'returned']
-
+          - name: customer_id
+            tests:
+              - not_null
+              - relationships:
+                  to: ref('stg_customers')
+                  field: customer_id
     ```
 
     </File>