Crashes when trying to run `perf`

[PabloAB]

Running asd or something not installed works fine. But trying to run perf make cnf to crash.

```
command-not-found version: 0.3
Python version: 3.8.10 final 0
Distributor ID: Ubuntu
Description: Ubuntu 20.04.5 LTS
Release: 20.04
Codename: focal
Exception information:

can only concatenate str (not "list") to str
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/CommandNotFound/util.py", line 23, in crash_guard
    callback()
  File "/usr/lib/command-not-found", line 95, in main
    if not cnf.advise(args[0], options.ignore_installed) and not options.no_failure_msg:
  File "/usr/lib/python3/dist-packages/CommandNotFound/CommandNotFound.py", line 378, in advise
    self.advice_multi_deb_package(command, packages, snaps)
  File "/usr/lib/python3/dist-packages/CommandNotFound/CommandNotFound.py", line 251, in advice_multi_deb_package
    pad = max([len(s[0]) for s in snaps+packages])
TypeError: can only concatenate str (not "list") to str
```