ptx: special char breaks it with "thread 'main' panicked at 'assertion failed: end <= s.len()', src/uu/ptx/src/ptx.rs:291:5"

With foo.txt containing:
```
it’s disabled
```
The char `’` is key here

```
ptx -G foo.txt
thread 'main' panicked at 'assertion failed: end <= s.len()', src/uu/ptx/src/ptx.rs:291:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```