pubsub/gcppubsub: Add an example showing how to call ModifyAckDeadline

[vangent]

Fixed #3507.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.24%. Comparing base (a93c109) to head (ce9c96a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3510   +/-   ##
=======================================
  Coverage   73.24%   73.24%           
=======================================
  Files         113      113           
  Lines       15052    15052           
=======================================
  Hits        11025    11025           
  Misses       3262     3262           
  Partials      765      765           

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