Fix minor typo in Trainer's documentation

[SamuelLarkin]

What does this PR do?

Fixes a minor typo in Trainer's documentation.

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• Was this discussed/agreed via a GitHub issue? (not for typos and docs)
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• Did you list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53%. Comparing base (f6fd046) to head (e3f9b22).
Report is 89 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (f6fd046) and HEAD (e3f9b22). Click for more details.

HEAD has 516 uploads less than BASE

Flag	BASE (f6fd046)	HEAD (e3f9b22)
python3.10	42	11
cpu	146	23
lightning	92	15
pytest	99	2
python3.8	31	6
lightning_fabric	27	0
examples	7	0
app	7	0
python3.11	53	6
gpu	4	2
tpu	1	0
pytorch_lightning	20	10
pytorch2.0	27	12
pytest-full	52	23
pytorch2.1	12	6
pytorch2.3	6	2
pytorch2.2	7	3
lightning_app	4	0

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #19969      +/-   ##
==========================================
- Coverage      84%      53%     -31%     
==========================================
  Files         426      418       -8     
  Lines       35284    35131     -153     
==========================================
- Hits        29620    18711   -10909     
- Misses       5664    16420   +10756