Fix missing ' in client documentation

[koole]

What do these changes do?

Fix a (very) small mistake in the documentation.

[asvetlov]

Thanks

[codecov-io]

Codecov Report

Merging #2152 into 2.2 will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              2.2    #2152   +/-   ##
=======================================
  Coverage   97.03%   97.03%           
=======================================
  Files          38       38           
  Lines        7685     7685           
  Branches     1341     1341           
=======================================
  Hits         7457     7457           
  Misses        104      104           
  Partials      124      124

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