Fix incorrect error message in TensorArray.scatter

This fix tries to fix the issue raised in 12403 where the
error message in TensorArray.scatter is incorrect. Specifically, in
```
      OP_REQUIRES(
          ctx, max_index < array_size,
          errors::InvalidArgument("Max scatter index must be <= array size (",
                                  max_index, " vs. ", array_size, ")"));
```

The `<= array size` in error message should be `< array size` as
the maximum value of index (0-based) should always be smaller than array size.

This fix fixes 12403.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>