fix.sh in checkoff-0.20.0 vs fix.sh in checkoff-0.21.0

@@ -186,10 +186,10 @@
   # without needing to have a machine that matches PLATFORM handy to
   # install those platform-specific gems on.'
   #
   # This affects nokogiri, which will try to reinstall itself in
   # Docker builds where it's already installed if this is not run.
-  for platform in x86_64-darwin-20 x86_64-linux
+  for platform in x86_64-darwin-21 x86_64-linux
   do
     grep "${platform:?}" Gemfile.lock >/dev/null 2>&1 || bundle lock --add-platform "${platform:?}"
   done
 }