FIX heisenfailure in doc/memory.rst

[ogrisel]

As seen on various recent PRs.

[codecov]

Codecov Report

Base: 93.90% // Head: 93.93% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (58979ef) compared to base (1dcbf00).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1396      +/-   ##
==========================================
+ Coverage   93.90%   93.93%   +0.03%     
==========================================
  Files          52       52              
  Lines        7300     7341      +41     
==========================================
+ Hits         6855     6896      +41     
  Misses        445      445              

Impacted Files	Coverage Δ
joblib/hashing.py	89.47% <0.00%> (-1.76%)	⬇️
joblib/test/test_parallel.py	96.93% <0.00%> (-0.12%)	⬇️
joblib/func_inspect.py	92.43% <0.00%> (+0.08%)	⬆️
joblib/memory.py	95.52% <0.00%> (+0.27%)	⬆️
joblib/_parallel_backends.py	93.38% <0.00%> (+0.36%)	⬆️
joblib/test/test_func_inspect.py	91.36% <0.00%> (+0.74%)	⬆️
joblib/_store_backends.py	91.58% <0.00%> (+0.81%)	⬆️
joblib/test/test_store_backends.py	94.54% <0.00%> (+3.11%)	⬆️

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[ogrisel]

PyPy is deadlocked after:

joblib/test/test_parallel.py::test_thread_bomb_mitigation[loky] PASSED   [ 96%]

this has been observed elsewhere and is not related to this PR.