Remove duplicate underline in B007 autofix message

[SigureMo]

fixes #3020

In the case of #3020, rename already contains an underscore (i.e. _i), so the error message contains two underscores (i.e. __i) and __ is lost because of rust-lang/annotate-snippets-rs#53. So the final output has no underscore (i.e. i)

The output after applying this fix is shown below:

test.py:1:5: B007 [*] Loop control variable `i` not used within loop body
  |
1 | for i in range(0, 100):
  |     ^ B007
  |
  = help: Rename unused `i` to `_i`

[charliermarsh]

Thanks!