docs: Fix exit code for NoCommitsFoundError

[mmartinortiz]

The source code indicates that the correct exit code is 3. See https://github.com/commitizen-tools/commitizen/blob/master/commitizen/exceptions.py#L10

Description

Update the documentation with the correct exit number

Checklist

• Add test cases to all the changes you introduce
• Run ./script/format and ./script/test locally to ensure this change passes linter check and test
• Test the changes on the local machine manually
• Update the documentation for the changes

The first three entries do not apply to this PR.

Expected behavior

The documentation shows the correct information.

[codecov]

Codecov Report

Merging #468 (308de9c) into master (1f37002) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #468   +/-   ##
=======================================
  Coverage   98.04%   98.04%           
=======================================
  Files          39       39           
  Lines        1431     1431           
=======================================
  Hits         1403     1403           
  Misses         28       28           

Flag	Coverage Δ
unittests	98.04% <ø> (ø)

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