docs(readme): zlib deprecation prevention

[Leafgard]

Access Z_SYNC_FLUSH is still possible but deprecated as you can see on Zlib documentation.

Previously, the constants were available directly from require('zlib'), for instance zlib.Z_NO_FLUSH. Accessing the constants directly from the module is currently still possible but is deprecated.

Prevented this deprecation by accessing Z_SYNC_FLUSH from constants namespace.

[codecov]

Codecov Report

Merging #119 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #119   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           80        80           
  Branches        29        29           
=========================================
  Hits            80        80           

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[niftylettuce]

v5.0.1 released to npm and GitHub

https://github.com/koajs/compress/releases/tag/v5.0.1