Typo in System.Data.DataColumn.xml (dotnet#7523)

Author: DavidMoore

xml/System.Data/DataColumn.xml

@@ -1122,7 +1122,7 @@ myDataTable.Rows[0][0] = newValue;
   
 -   Its <xref:System.Data.DataColumn.ColumnMapping%2A> can be only set to `MappingType.Element`.  
   
--   The class that implements the column's data type must be must be marked with the <xref:System.SerializableAttribute>, and if necessary implement the <xref:System.Runtime.Serialization.ISerializable> or <xref:System.Xml.Serialization.IXmlSerializable> interface.  
+-   The class that implements the column's data type must be marked with the <xref:System.SerializableAttribute>, and if necessary implement the <xref:System.Runtime.Serialization.ISerializable> or <xref:System.Xml.Serialization.IXmlSerializable> interface.  
   
 -   Support for change tracking is limited. To utilize the <xref:System.Data.DataTable> class' change tracking mechanism, the class that implements the column's data type must either implement the <xref:System.ComponentModel.IChangeTracking> interface, or take over the responsibility for informing the <xref:System.Data.DataRow> when the column value has been modified, either by calling <xref:System.Data.DataRow.SetModified%2A> on the row or by assigning the column value object to a separately instantiated column value object.