Fix clap doc-comment

[Long0x0]

clap always removes the last single period from a doc-comment.

before:

~$ lsd --help
  -a, --all                          Do not ignore entries starting with

after:

~$ lsd --help
  -a, --all                          Do not ignore entries starting with .

---

TODO

• Use cargo fmt
• Add necessary tests
• Update default config/theme in README (if applicable)
• Update man page at lsd/doc/lsd.md (if applicable)

[muniu-bot]

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[outloudvi]

To fix this, Clap also has a flag called verbatim_doc_comment to avoid removing trailing dots and so on, but solution of this PR seems more specific (and not breaking other benefits of Clap's pre-processing).