fixing error in xval function in gumbel.jl

[ajshephard]

There is a typo in the xval function in gumbel.jl. This closes #1858

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 85.60%. Comparing base (f852803) to head (6d2ebf5).

Files	Patch %	Lines
src/univariate/continuous/gumbel.jl	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1859      +/-   ##
==========================================
- Coverage   85.96%   85.60%   -0.37%     
==========================================
  Files         144      144              
  Lines        8647     8646       -1     
==========================================
- Hits         7433     7401      -32     
- Misses       1214     1245      +31     

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[andreasnoack]

Please add a test as well. Is xval generally not tested across distributions?

[devmotion]

As far as I can tell, for Gumbel currently xval is just not used anywhere. I assume for most distributions it is tested implicitly since it is used in a function such as quantile.

So, I guess either one could remove xval for Gumbel or one could define e.g. quantile in terms of xval.

[devmotion]

Thank you!