Fixed a silly bug within document.InsertTable(int, int).

This bug caused a Table to be inserted "inside" the first element of a document's XML instead of "as" the first element.

DocX/Container.cs

         public Table InsertTable(int rowCount, int coloumnCount)
         {
             XElement newTable = HelperFunctions.CreateTable(rowCount, coloumnCount);
-            Xml.Elements().First().Add(newTable);
+            Xml.Add(newTable);
 
             return new Table(Document, newTable);
         }