docs(README): fix link to documentation

[woile]

Description

Correct link to docs in README

Checklist

• Update the documentation for the changes

Expected behavior

Additional context

Closes #245

[codecov]

Codecov Report

Merging #246 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #246      +/-   ##
==========================================
+ Coverage   96.58%   96.60%   +0.01%     
==========================================
  Files          33       33              
  Lines         908      912       +4     
==========================================
+ Hits          877      881       +4     
  Misses         31       31              

Flag	Coverage Δ
#unittests	96.60% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
commitizen/commands/bump.py	90.58% <ø> (ø)
commitizen/git.py	95.55% <ø> (+0.04%)	⬆️
commitizen/__version__.py	100.00% <100.00%> (ø)
commitizen/commands/check.py	100.00% <100.00%> (ø)
commitizen/cz/base.py	100.00% <0.00%> (ø)

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