Update CRAN URLs in README.md

[pwep]

• Use RJafroc as package name in URLs in the README
• fixes CRAN badge and associated link to CRAN info page

[codecov[bot]]

Codecov Report

Merging #57 into developer will not change coverage. The diff coverage is n/a.

@@            Coverage Diff             @@
##           developer      #57   +/-   ##
==========================================
  Coverage      69.78%   69.78%           
==========================================
  Files             79       79           
  Lines           9809     9809           
==========================================
  Hits            6845     6845           
  Misses          2964     2964           

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