Fix missing comma in _benchmark.__all__

[maurycy]

No description provided.

[maurycy]

I noticed this while familiarizing with the code but uvx ruff check catches it as well:

23:38:27.527666000PM CEST maurycy@gimel /Users/maurycy/src/pyperformance % uvx ruff check pyperformance/_benchmark.py
F822 Undefined name `Benchmarkcheck_name` in `__all__`
 --> pyperformance/_benchmark.py:4:5
  |
2 |   __all__ = [
3 |       'BenchmarkSpec',
4 | /     'Benchmark'
5 | |     'check_name',
  | |________________^
6 |       'parse_benchmark',
7 |   ]
  |

Maybe it's a good idea to run it over the whole project?

[AA-Turner]

Rather apposite that this was PR #404