fix: update Makefile verify-mdlint

[stillfox-lee]

Type of change:

• Bugfix
• New feature provided
• Improve performance
• Backport patches

What this PR does / why we need it:

issue: #1110
Update Makefile's verify-mdlint. Ignore CHANGELOG.md.

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[codecov-commenter]

Codecov Report

Merging #1126 (8fffbd9) into master (0e1f8d4) will not change coverage.
The diff coverage is n/a.

❗ Current head 8fffbd9 differs from pull request most recent head 06c69ca. Consider uploading reports for the commit 06c69ca to get more accurate results

@@           Coverage Diff           @@
##           master    #1126   +/-   ##
=======================================
  Coverage   30.38%   30.38%           
=======================================
  Files          81       81           
  Lines        9390     9390           
=======================================
  Hits         2853     2853           
  Misses       6222     6222           
  Partials      315      315           

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[tao12345666333]

LGTM

thanks!

[lijing-21]

Hi @stillfox-lee , thanks for the contribution!

Here is the Contributor T-shirt form[1], if you're interested, kindly take a look :)

[1] https://github.com/apache/apisix/blob/master/CONTRIBUTING.md#contributor-t-shirt