Fix type error in '#info-r'/-2

The size of a record is the number of fields in the record plus 1.

This is the error we got when running Dialyzer

```
Invalid type specification for function XXX:'#info-YYY'/1. The success typing is ('fields' | 'size') -> [] | 1
```

and this is the function specification generated by exprecs

```
-spec '#info-YYY'(fields) -> [];
                          (size) -> 0.
```