Change SpreadSheetML namespace to SpreadsheetML

**Description**

The `DocumentFormat.OpenXml.Office.SpreadSheetML` namespace should be changed to `DocumentFormat.OpenXml.Office.SpreadsheetML` (with lowercase s)

**Information**

- .NET Target: any
- DocumentFormat.OpenXml Version: 2.15.0+

**Expected**

Spreadsheet should be one word.
