doc: fix markdown errors in api-plugin doc

[seeflood]

What this PR does:
Fix markdown errors in api_plugin/design

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov]

Codecov Report

Merging #433 (478c991) into main (428540f) will not change coverage.
The diff coverage is n/a.

❗ Current head 478c991 differs from pull request most recent head d454bc0. Consider uploading reports for the commit d454bc0 to get more accurate results

@@           Coverage Diff           @@
##             main     #433   +/-   ##
=======================================
  Coverage   60.18%   60.18%           
=======================================
  Files         117      117           
  Lines        6252     6252           
=======================================
  Hits         3763     3763           
  Misses       2128     2128           
  Partials      361      361           

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[seeflood]

now it's ready for review: