fixed a missing description of the MSRV option for manual_repeat_n

[lapla-cogito]

I added manual_repeat_n lint before, which has the MSRV option. However, I forgot to add this lint to the lint configuration.

changelog: none

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[y21]

Ah nice catch, thanks!
r? y21
(stealing since this is a pretty small PR and I just looked through it)

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