Fix comment for File.Exists (#85648)

Author: SiqiLu

src/libraries/System.Private.CoreLib/src/System/IO/File.cs

@@ -88,7 +88,7 @@ public static void Delete(string path)
         // Tests whether a file exists. The result is true if the file
         // given by the specified path exists; otherwise, the result is
         // false.  Note that if path describes a directory,
-        // Exists will return true.
+        // Exists will return false.
         public static bool Exists([NotNullWhen(true)] string? path)
         {
             try