Fix code sample for "The init keyword" docs (#43718)

The validation logic check the actual new value instead of old stored property value.

Author: snechaev

docs/csharp/language-reference/keywords/snippets/InitExample5.cs

@@ -5,7 +5,7 @@ public int YearOfBirth
         get;
         init
         {
-            field = (YearOfBirth <= DateTime.Now.Year)
+            field = (value <= DateTime.Now.Year)
                 ? value
                 : throw new ArgumentOutOfRangeException(nameof(value), "Year of birth can't be in the future");
         }