Fix `--interval` option docs in `pr checks`

Just noticed the usage doc for the `--interval` flag in `pr checks` shows `--watch` as placeholder:

```
$ gh pr checks --help
...
  -i, --interval --watch   Refresh interval in seconds when using --watch flag (default 10)
...
```

This has to be fixed into something like this:

```
  -i, --interval int   Refresh interval in seconds when using --watch flag (default 10)
```
