Fix wrong link for the crendentials page in MDN in rest.md

[petetnt]

The credentials link was pointing to headers instead of credentials

[codecov-io]

Codecov Report

Merging #1250 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1250   +/-   ##
======================================
  Coverage    95.6%   95.6%           
======================================
  Files          22      22           
  Lines        1116    1116           
  Branches      174     162   -12     
======================================
  Hits         1067    1067           
  Misses         44      44           
  Partials        5       5

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