Update sources model with clear comment on what freshness block is used by the customers table

[chisom-mbama]

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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