Amend is_euler_prp() docstring

[skirpichev]

See e.g. https://en.wikipedia.org/wiki/Solovay%E2%80%93Strassen_primality_test

Closes #427

[codecov-commenter]

Codecov Report

Merging #428 (3dd20b2) into master (2beaae2) will not change coverage. The diff coverage is n/a.

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@@           Coverage Diff           @@
##           master     #428   +/-   ##
=======================================
  Coverage   83.63%   83.63%           
=======================================
  Files          49       49           
  Lines       11715    11715           
  Branches     2190     2190           
=======================================
  Hits         9798     9798           
  Misses       1917     1917           

Files Changed	Coverage Δ
src/gmpy_mpz_prp.c	79.90% <ø> (ø)

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[casevh]

Thanks!