Make static increment and decrement operator immutable

The call to the static operator a++ already do the storing in a of the new value. And doing `++operand.numerator ` will means `b = a++` will have b receive a mutated value and calls to a method with `Console.WriteLine(myFraction++);` would have the new mutated value instead of the old value.
So I'm changing the line to operand.numerator + 1;

Author: Esurnir

docs/csharp/language-reference/operators/snippets/shared/OperatorOverloading.cs

@@ -36,10 +36,10 @@ public Fraction(int numerator, int denominator)
 
     // Define increment and decrement to add 1/den, rather than 1/1.
     public static Fraction operator ++(Fraction operand)
-        => new Fraction(++operand.numerator, operand.denominator);
+        => new Fraction(operand.numerator + 1, operand.denominator);
 
     public static Fraction operator --(Fraction operand) =>
-        new Fraction(--operand.numerator, operand.denominator);
+        new Fraction(operand.numerator - 1, operand.denominator);
 
     public override string ToString() => $"{numerator} / {denominator}";