Update Dart documentation (2.19.6)

Author: simon04

lib/docs/scrapers/dart.rb

@@ -21,7 +21,7 @@ class Dart < FileScraper
     HTML
 
     version '2' do
-      self.release = '2.18.5'
+      self.release = '2.19.6'
       self.base_url = "https://api.dart.dev/stable/#{release}/"
     end