Fix typo in reference-core.rst

[samsamoa]

Normally I wouldn't make such a trivial PR, but this is such an excellent quote that I think it deserves to be fixed.

[codecov]

Codecov Report

Merging #2712 (47c1850) into master (315dbed) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2712   +/-   ##
=======================================
  Coverage   98.88%   98.88%           
=======================================
  Files         113      113           
  Lines       16482    16482           
  Branches     3000     3000           
=======================================
  Hits        16299    16299           
  Misses        126      126           
  Partials       57       57           

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