[DOC] Use URI#read instead of URI.open for better compatibility

- URI.open is available since 2.5.0, but URI#read is available since 1.8.0.
- Fix `open` without `close`.
- ref https://github.com/rurema/doctree/pull/2433

kernel.rb

@@ -105,7 +105,7 @@ module Kernel
   #     require 'json'
   #
   #     construct_url(arguments).
-  #       then {|url| URI.open(url).read }.
+  #       then {|url| URI(url).read }.
   #       then {|response| JSON.parse(response) }
   #
   #  When called without block, the method returns +Enumerator+,
@@ -138,7 +138,7 @@ module Kernel
   #     require 'json'
   #
   #     construct_url(arguments).
-  #       then {|url| URI.open(url).read }.
+  #       then {|url| URI(url).read }.
   #       then {|response| JSON.parse(response) }
   #
   def yield_self