fix annotation regex for flake8 (#26694)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26694
Previously we would not properly populate `errorDesc` for:
```
./torch/jit/__init__.py:13:1: F401 'torch.nn.ModuleList' imported but unused
```
because we wanted only letters and spaces. Be more permissive
Test Plan: Imported from OSS
Differential Revision: D17551999
Pulled By: suo
fbshipit-source-id: b82567df1fa3c9729e7427dc3461bedfb40933dc
