Update rest endpoint names in README

[jotok]

Is your feature request related to a problem? Please provide an existing Issue # , or describe.
Update the README to replace the legace /_opendistro REST resource with /_plugins.

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff

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[codecov-commenter]

Codecov Report

Merging #15 (6c16f27) into main (0ada819) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main      #15   +/-   ##
=========================================
  Coverage     72.09%   72.09%           
  Complexity      346      346           
=========================================
  Files            44       44           
  Lines          2473     2473           
  Branches        155      155           
=========================================
  Hits           1783     1783           
  Misses          585      585           
  Partials        105      105           

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