Fix doc formatting of LinearRegressionModel #4418

Job	Run time
docs	12m 31s
lint	1m 14s
check-examples (03-FFT-examples.ipynb)	6m 8s
tests (macos-latest, 3.9, all)	44m 28s
check-examples (04-RNN-examples.ipynb)	12m 10s
tests (ubuntu-latest, 3.9, all)	55m 8s
check-examples (00-quickstart.ipynb)	8m 44s
check-examples (02-data-processing.ipynb)	6m 19s
check-examples (01-multi-time-series-and-covariates.ipynb)	8m 54s
	2h 35m 36s

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