Refer to `logprior` in `logjoint`'s doc

[sunxd3]

Feels like a typo, c.f. logprior and loglikelihood's docs.

[coveralls]

Pull Request Test Coverage Report for Build 9032282777

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 78.547%

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Totals
Change from base Build 9029996782:	0.03%
Covered Lines:	2790
Relevant Lines:	3552

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💛 - Coveralls