fix docs typo

[trim21]

Description

fix typo in docs

Checklist

• Update the documentation for the changes

Expected behavior

Steps to Test This Pull Request

cz ch --unreleased_version="v1.0.0"
usage: cz [-h] [--debug] [-n NAME]
{init,commit,c,ls,example,info,schema,bump,changelog,ch,check,version}
...
cz: error: unrecognized arguments: --unreleased_version=v1.0.0

Additional context

[Lee-W]

LGTM. Thanks for your contribution!

[codecov]

Codecov Report

Merging #377 (f2df460) into master (8638512) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #377   +/-   ##
=======================================
  Coverage   97.60%   97.60%           
=======================================
  Files          39       39           
  Lines        1378     1378           
=======================================
  Hits         1345     1345           
  Misses         33       33           

Flag	Coverage Δ
unittests	97.60% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
commitizen/__version__.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ba94e3f...f2df460. Read the comment docs.