Fix doc: open file with "wb" mode will get an error in python 3

[yandongxu]

python: 3.6.0rc2
scrapy: 1.3.0

output:

TypeError: a bytes-like object is required, not 'str'

[codecov-io]

Codecov Report

Merging #2729 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #2729   +/-   ##
======================================
  Coverage    84.7%   84.7%           
======================================
  Files         162     162           
  Lines        9125    9125           
  Branches     1354    1354           
======================================
  Hits         7729    7729           
  Misses       1145    1145           
  Partials      251     251

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[kmike]

A good catch, thanks!