Update source/use-cases/pre-aggregated-reports.txt

To retrieve several months of daily data：
...     { 'metadata.date': 1, 'hourly': 1 } },
new:
...     { 'metadata.date': 1, 'daily': 1 } },

Author: chen56

source/use-cases/pre-aggregated-reports.txt

@@ -531,7 +531,7 @@ query:
    ...         'metadata.date': { '$gte': dt1, '$lte': dt2 },
    ...         'metadata.site': 'site-1',
    ...         'metadata.page': '/index.html'},
-   ...     { 'metadata.date': 1, 'hourly': 1 } },
+   ...     { 'metadata.date': 1, 'daily': 1 } },
    ...     sort=[('metadata.date', 1)])
 
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