Fix TypeError messages

## current
```bash
$ mirb
mirb - Embeddable Interactive Ruby Shell

This is a very early version, please test and report errors.
Thanks :)

> [][[1,2,3]]
TypeError: can't convert 123 into Integer
> [1,2,3] & 4
TypeError: can't convert to Array
>
```

## fixed
```bash
$ mirb
mirb - Embeddable Interactive Ruby Shell

This is a very early version, please test and report errors.
Thanks :)

> [][[1,2,3]]
TypeError: can't convert Array into Integer
> [1,2,3] & 4
TypeError: can't convert Fixnum into Array
>
```