Use the session in async subs example

[jaylett]

client.subscribe() is not async safe, but session.subscribe() is.

[leszekhanusz]

Thank you!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7fb869a) to head (e054045).
⚠️ Report is 51 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##            master      #585    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           38        40     +2     
  Lines         2908      3311   +403     
==========================================
+ Hits          2908      3311   +403     

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