Fixed #789 Typo in "how to add custom feature"

[seanlaw]

Fixed minor typo from set_propert to set_property

[codecov-io]

Codecov Report

Merging #790 (5bee6d1) into main (93f3a57) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #790   +/-   ##
=======================================
  Coverage   95.89%   95.89%           
=======================================
  Files          18       18           
  Lines        1777     1777           
  Branches      347      347           
=======================================
  Hits         1704     1704           
  Misses         36       36           
  Partials       37       37           

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[nils-braun]

Thanks! Nice catch