Update sources model with clear comment on what freshness block is us…

…ed by the customers table (#5687)

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

I am making updates to a comment in the model, to provide clarity on
which exact freshness parameter is used by the customers table.

It was unclear if the freshness used by the customers table is the one
defined above for the orders table, or the default freshness defined in
the source block; since both freshness parameters are 'defined above'

- [x] Updated comment needs to be reviewed for accuracy


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---------

Co-authored-by: Mirna Wong <[email protected]>

website/docs/docs/build/sources.md

@@ -155,7 +155,7 @@ sources:
           warn_after: {count: 6, period: hour}
           error_after: {count: 12, period: hour}
 
-      - name: customers # this will use the freshness defined above
+      - name: customers # this inherits the default freshness defined in the jaffle_shop source block at the beginning
 
 
       - name: product_skus