docs(bump): update cli tag flag format

[ghost]

Description

Change in bump documentation.

The flag in the CLI for tag format is --tag-format and not --tag_format.

[codecov]

Codecov Report

Merging #333 (1a33099) into master (b435289) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #333      +/-   ##
==========================================
+ Coverage   97.17%   97.20%   +0.02%     
==========================================
  Files          35       35              
  Lines         991     1000       +9     
==========================================
+ Hits          963      972       +9     
  Misses         28       28              

Flag	Coverage Δ
unittests	97.20% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
commitizen/__version__.py	100.00% <100.00%> (ø)
commitizen/commands/changelog.py	97.40% <100.00%> (+0.10%)	⬆️
commitizen/cz/base.py	100.00% <100.00%> (ø)
commitizen/cz/customize/customize.py	94.11% <100.00%> (+0.50%)	⬆️
commitizen/defaults.py	100.00% <100.00%> (ø)

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