[Docs] Fix typo in Trainer.test()

[JamesTrick]

What does this PR do?

This is an extremely small PR fixing a typo in the Trainer.test() docs example.

Currently, it reads:

Trainer.test(test_dataloader=test_dataloader)

It's currently missing the 's', it should read:

Trainer.test(test_dataloaders=test_dataloader)

Fixes: No issue found/small typo.

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[codecov]

Codecov Report

Merging #5226 (1320429) into master (5651c9c) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #5226   +/-   ##
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- Coverage      93%     93%   -0%     
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  Files         134     134           
  Lines        9945    9942    -3     
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- Hits         9259    9256    -3     
  Misses        686     686