[Docs] Add experiment_key argument to online mode example for CometLogger

[c00k1ez]

What does this PR do?

Small fix for documentation. Added experiment_key argument to online mode example for CometLogger.

Discussed this issue on slack.

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[codecov]

Codecov Report

Merging #4997 (5bc9e17) into master (cb28f79) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #4997   +/-   ##
======================================
  Coverage      93%     93%           
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  Files         130     130           
  Lines        9547    9547           
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  Hits         8871    8871           
  Misses        676     676           

[awaelchli]

<3