Fix broken link in README

[danielaparker]

The link in the README file to the latest JSON Schema Core specification, http://json-schema.org/latest/json-schema-core.html, is a dead link, it no longer exists. I changed it to https://json-schema.org/specification

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.15%. Comparing base (48ca3c2) to head (f93b8d4).
Report is 9 commits behind head on master.

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1029      +/-   ##
============================================
+ Coverage     78.90%   79.15%   +0.24%     
- Complexity     1965     1986      +21     
============================================
  Files           172      174       +2     
  Lines          6352     6427      +75     
  Branches       1255     1261       +6     
============================================
+ Hits           5012     5087      +75     
+ Misses          867      864       -3     
- Partials        473      476       +3     

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[stevehu]

@danielaparker Thanks for your help.