fix the shape bug in `LKJCovariancePrior` #754

Job	Run time
run_linter	38s
run_examples	5m 57s
run_unit_tests (stable, no-extras)	1m 39s
run_unit_tests (main, no-extras)	3m 17s
run_unit_tests (stable, with-extras)	2m 17s
run_unit_tests (main, with-extras)	4m 5s
	17m 53s

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