Plackett copula documentation

[Santymax98]

Hi!!, i added information about the Plackett copula, let me know if anything else is missing

[codecov]

Codecov Report

Merging #43 (28a2dde) into main (815dc84) will not change coverage.
The diff coverage is n/a.

❗ Current head 28a2dde differs from pull request most recent head adf8ba2. Consider uploading reports for the commit adf8ba2 to get more accurate results

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##             main      #43   +/-   ##
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  Coverage   77.40%   77.40%           
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  Files          22       22           
  Lines         332      332           
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  Hits          257      257           
  Misses         75       75           

Files	Coverage Δ
src/MiscellaneousCopulas/PlackettCopula.jl	94.28% <ø> (ø)

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[lrnv]

The triple ticks documentation needs to be right before the object (i think). Otherwise this is perfect, thank you very much !