Update updating-snomed-and-extensions.md

[JonZammit]

I updated this documentation to reflect the correct end point for code system.

[codecov]

Codecov Report

Merging #84 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #84   +/-   ##
=========================================
  Coverage     66.67%   66.67%           
  Complexity     2335     2335           
=========================================
  Files           243      243           
  Lines          9812     9812           
  Branches       1161     1161           
=========================================
  Hits           6542     6542           
  Misses         2770     2770           
  Partials        500      500

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