Update erlang.jl docstring

[theplatters]

Fixed the mismatch in the parameter names and gave the concrete pdf, so the reference to wikipedia doesn't create confusion.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.62%. Comparing base (aad64af) to head (900584b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1915      +/-   ##
==========================================
- Coverage   85.99%   85.62%   -0.38%     
==========================================
  Files         144      141       -3     
  Lines        8685     8638      -47     
==========================================
- Hits         7469     7396      -73     
- Misses       1216     1242      +26     

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[mschauer]

Thank you, it is desirable to use the same parameter names as wikipedia, so here k and beta?

[theplatters]

Yeha ok either alpha and theta to be consitent with the gamma distribution, or k and beta to be consitent with wikipedia.