Update BigInteger.xml (#10021)

Author: tfenise

xml/System.Numerics/BigInteger.xml

@@ -2218,7 +2218,7 @@ This method returns 0 if the value of current object is equal to <xref:System.Nu
 > [!NOTE]
 >  Computing the greatest common divisor of very large values of `left` and `right` can be a very time-consuming operation.
 
- The value returned by the <xref:System.Numerics.BigInteger.GreatestCommonDivisor%2A> method is always positive regardless of the sign of the `left` and `right` parameters.
+ The value returned by the <xref:System.Numerics.BigInteger.GreatestCommonDivisor%2A> method is always positive (including zero) regardless of the sign of the `left` and `right` parameters.