Update tmpdir.rst - very trivial

[ykantor]

No description provided.

[nicoddemus]

Thanks!

[codecov]

Codecov Report

Merging #4201 into master will increase coverage by 1.13%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #4201      +/-   ##
=========================================
+ Coverage   94.67%   95.8%   +1.13%     
=========================================
  Files         109     109              
  Lines       24247   24257      +10     
  Branches     2387    2390       +3     
=========================================
+ Hits        22955   23240     +285     
+ Misses        985     721     -264     
+ Partials      307     296      -11

Flag	Coverage Δ
#docs	28.5% <ø> (-0.01%)	⬇️
#doctesting	28.5% <ø> (-0.01%)	⬇️
#linting	28.5% <ø> (-0.01%)	⬇️
#linux	95.58% <ø> (+1.88%)	⬆️
#nobyte	91.88% <ø> (+0.57%)	⬆️
#numpy	92.9% <ø> (+1%)	⬆️
#pexpect	41.27% <ø> (?)
#py27	93.94% <ø> (+1.47%)	⬆️
#py34	92.22% <ø> (+0.02%)	⬆️
#py35	92.23% <ø> (+0.64%)	⬆️
#py36	93.89% <ø> (+0.92%)	⬆️
#py37	92.26% <ø> (?)
#trial	92.9% <ø> (+1%)	⬆️
#windows	94.04% <ø> (ø)	⬆️
#xdist	93.77% <ø> (+0.47%)	⬆️

Impacted Files	Coverage Δ
testing/code/test_code.py	94.11% <0%> (-1.34%)	⬇️
testing/code/test_source.py	97.02% <0%> (ø)	⬆️
testing/test_warnings.py	98.55% <0%> (+0.05%)	⬆️
src/_pytest/assertion/rewrite.py	95.68% <0%> (+0.16%)	⬆️
src/_pytest/pytester.py	87.3% <0%> (+0.72%)	⬆️
testing/test_terminal.py	100% <0%> (+0.78%)	⬆️
src/_pytest/terminal.py	92.84% <0%> (+1.39%)	⬆️
src/_pytest/helpconfig.py	99.06% <0%> (+1.86%)	⬆️
testing/python/integration.py	91.48% <0%> (+2.12%)	⬆️
src/_pytest/doctest.py	96.78% <0%> (+2.4%)	⬆️
... and 3 more

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