Fix example usage of ActiveRecord::Base.establish_connection

`ActiveRecord::Base.establish_connection` accepts a single symbol argument to specify a named connection; a single string argument appears to be interpreted as a connection URI

activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb

@@ -825,7 +825,7 @@ def checkout_and_verify(c)
     #   end
     #
     #   class Book < ActiveRecord::Base
-    #     establish_connection "library_db"
+    #     establish_connection :library_db
     #   end
     #
     #   class ScaryBook < Book