small fix

Author: Nick-Nuon

src/UTF8_validation.cs

@@ -361,8 +361,8 @@ private Vector256<byte> check_multibyte_lengths(Vector256<byte> input, Vector256
 
             private Vector256<byte> must_be_2_3_continuation(Vector256<byte> prev2, Vector256<byte> prev3)
             {
-                Vector256<byte> is_third_byte = Avx2.SubtractSaturate(prev2, Vector256.Create((byte)(0b11100000u - 1)));
-                Vector256<byte> is_fourth_byte = Avx2.SubtractSaturate(prev3, Vector256.Create((byte)(0b11110000u - 1)));
+                Vector256<byte> is_third_byte = Avx2.SubtractSaturate(prev2, Vector256.Create((byte)(0b11100000u - 0x80)));
+                Vector256<byte> is_fourth_byte = Avx2.SubtractSaturate(prev3, Vector256.Create((byte)(0b11110000u - 0x80)));
 
                 Vector256<byte> combined = Avx2.Or(is_third_byte, is_fourth_byte);
                 return combined;