DOC use proper math symbol for subset

[sadrasabouri]

Reference Issue

close #862

What does this implement/fix? Explain your changes.

\in changed to \subset in doc/under_sampling.rst protype section.

P.n: I'm not sure which branch should I PR to but I think master is the best choice

[codecov]

Codecov Report

Merging #867 (ec112f1) into master (56eefdf) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #867   +/-   ##
=======================================
  Coverage   98.84%   98.84%           
=======================================
  Files          93       93           
  Lines        6064     6064           
  Branches      508      508           
=======================================
  Hits         5994     5994           
  Misses         69       69           
  Partials        1        1           

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[glemaitre]

Thanks @sadrasabouri