docs: fix highlight of code example in "Providing values for nested data" #4211

Job	Run time
mypy	2m 8s
slotscheck	1m 31s
pyright	1m 48s
Test server integration (uvicorn<0.27.0)	1m 32s
validate	16s
Test server integration (uvicorn>=0.27.0)	1m 33s
Test Minimal Application with Base Dependencies	1m 27s
Test Minimal Pydantic 1 application	1m 38s
test-platform-compat	0s
test (3.10) / test	2m 42s
test (3.11) / test	2m 25s
test (3.12) / test	4m 16s
test (3.8) / test	3m 49s
test (3.9) / test	2m 45s
build-docs	3m 59s
codeql	3m 6s
upload-test-coverage	22s
sonar	1m 38s
	36m 55s

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