Update 06a-data-warehouse-load.md

[luconsta]

The first query from section "Run analytical queries" is wrong:

```sql
SELECT c.CustomerName, SUM(s.UnitPrice * s.Quantity) AS TotalSales
FROM Sales.Fact_Sales s
JOIN Sales.Dim_Customer c
ON s.SalesOrderNumber = c.SalesOrderNumber
WHERE YEAR(s.OrderDate) = 2021
GROUP BY c.CustomerName
ORDER BY TotalSales DESC;
```

The line "ON s.SalesOrderNumber = c.SalesOrderNumber" should be changed with "ON s.CustomerID = c.CustomerID" as the Fact_Sales and Dim_Customer are related on CustomerID not on SalesOrderNumber

Lab: 06a

Fixes # .

Changes proposed in this pull request:

-

-

[Mihai-Ac]

Hello @luconsta

Change was made with another commit. Thank you, Mihai