fix byte count in ToUInt128 description (#11805)

Author: StuartMosquera

xml/System/BitConverter.xml

@@ -2780,7 +2780,7 @@
       <Docs>
         <param name="value">An array of bytes.</param>
         <param name="startIndex">The starting position within <paramref name="value" />.</param>
-        <summary>Returns a 128-bit unsigned integer converted from four bytes at a specified position in a byte array.</summary>
+        <summary>Returns a 128-bit unsigned integer converted from sixteen bytes at a specified position in a byte array.</summary>
         <returns>A 128-bit unsigned integer formed by sixteen bytes beginning at <paramref name="startIndex" />.</returns>
         <remarks>To be added.</remarks>
         <exception cref="T:System.ArgumentException">