Pytrf findstr 'bed' output format not working

With simple fasta file as following: 

```
>seq1
ATATATATATATATATATATATATATATATATATAT
>seq2
CAGCAGCAGCAGCAGCAGCAGCAGCAGCAGCAGCAG
```

-> pytrf findstr is working with all output format types but not with bed. I get the following error:

```
TypeError: 'tuple' object does not support item assignment
```

Version: pytrf 1.4.2