Update README regarding access-token endpoint

[frederikprijck]

Changed the wording of client to client-side, as from an OIDC perspective, the entire nextjs app is a client.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.68%. Comparing base (56888c3) to head (a7ff7d5).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2044   +/-   ##
=======================================
  Coverage   79.68%   79.68%           
=======================================
  Files          21       21           
  Lines        1925     1925           
  Branches      316      316           
=======================================
  Hits         1534     1534           
  Misses        385      385           
  Partials        6        6           

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