Change where clause in unique_key example (#5897)

## What are you changing in this pull request and why?
The max statement in the where clause for the unique_key example
referred to event_time, but event_time isn't in the select statement. So
this should be updated to max(date_day).

## Checklist
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Co-authored-by: Matt Shaver <60105315+matthewshaver@users.noreply.github.com>

website/docs/docs/build/incremental-models.md

@@ -142,7 +142,7 @@ from {{ ref('app_data_events') }}
 
   -- this filter will only be applied on an incremental run
   -- (uses >= to include records arriving later on the same day as the last run of this model)
-  where date_day >= (select coalesce(max(event_time), '1900-01-01') from {{ this }})
+  where date_day >= (select coalesce(max(date_day), '1900-01-01') from {{ this }})
 
 {% endif %}