ARROW-5262: [Python] Fix typo

[Ktakuya332C]

install is necessary to install the libraries with a requirements file.
Thank you for your review.

[codecov-io]

Codecov Report

Merging #4257 into master will decrease coverage by 24.43%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #4257       +/-   ##
===========================================
- Coverage   88.27%   63.84%   -24.44%     
===========================================
  Files         763      375      -388     
  Lines       93723    53145    -40578     
  Branches     1251        0     -1251     
===========================================
- Hits        82735    33928    -48807     
- Misses      10875    19217     +8342     
+ Partials      113        0      -113

Impacted Files	Coverage Δ
cpp/src/arrow/util/memory.h	0% <0%> (-100%)	⬇️
cpp/src/parquet/hasher.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/extension_type.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/util/sse-util.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/flight/server.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/json/chunker.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/util/uri.cc	0% <0%> (-100%)	⬇️
cpp/src/arrow/flight/client_auth.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/compute/logical_type.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/compute/kernels/filter.cc	0% <0%> (-100%)	⬇️
... and 579 more

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[jorisvandenbossche]

Thanks!