fix typo in tmpdir documentation (trivial)

[vbarbaresi]

🤷‍♂️

[codecov]

Codecov Report

Merging #4205 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4205      +/-   ##
==========================================
+ Coverage   95.79%   95.81%   +0.02%     
==========================================
  Files         109      109              
  Lines       24257    24257              
  Branches     2390     2390              
==========================================
+ Hits        23236    23242       +6     
+ Misses        721      720       -1     
+ Partials      300      295       -5

Flag	Coverage Δ
#docs	28.5% <ø> (ø)	⬆️
#doctesting	28.5% <ø> (ø)	⬆️
#linting	28.5% <ø> (ø)	⬆️
#linux	95.58% <ø> (+0.01%)	⬆️
#nobyte	91.88% <ø> (ø)	⬆️
#numpy	92.91% <ø> (ø)	⬆️
#pexpect	41.27% <ø> (ø)	⬆️
#py27	93.94% <ø> (+0.52%)	⬆️
#py34	92.22% <ø> (+0.03%)	⬆️
#py35	92.23% <ø> (+0.03%)	⬆️
#py36	93.9% <ø> (ø)	⬆️
#py37	92.26% <ø> (+0.03%)	⬆️
#trial	92.91% <ø> (ø)	⬆️
#windows	94.05% <ø> (ø)	⬆️
#xdist	93.78% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
src/_pytest/assertion/rewrite.py	95.68% <0%> (+0.16%)	⬆️
src/_pytest/capture.py	93.82% <0%> (+0.45%)	⬆️
testing/python/integration.py	91.48% <0%> (+2.12%)	⬆️

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[nicoddemus]

Thanks @vbarbaresi!