Fix minor documentation error for IFloatingPoint<TSelf>.Round(TSelf, int, MidpointRounding) (dotnet#9122)

Author: hamarb123

xml/System.Numerics/IFloatingPoint`1.xml

@@ -351,7 +351,7 @@
         <param name="x">The value to round.</param>
         <param name="digits">The number of fractional digits to which <paramref name="x" /> should be rounded.</param>
         <param name="mode">The mode under which <paramref name="x" /> should be rounded.</param>
-        <summary>Rounds a value to a specified number of fractional-digits using the default rounding mode (<see cref="F:System.MidpointRounding.ToEven" />).</summary>
+        <summary>Rounds a value to a specified number of fractional digits using the specified rounding mode.</summary>
         <returns>The result of rounding <paramref name="x" /> to <paramref name="digits" /> fractional-digits using <paramref name="mode" />.</returns>
         <remarks>To be added.</remarks>
       </Docs>