use ruby 2.3.3 also in the update guide [ci skip]

doc/update/8.15-to-8.16.md

@@ -32,9 +32,9 @@ Download and compile Ruby:
 
 ```bash
 mkdir /tmp/ruby && cd /tmp/ruby
-curl --remote-name --progress https://cache.ruby-lang.org/pub/ruby/2.3/ruby-2.3.1.tar.gz
-echo 'c39b4001f7acb4e334cb60a0f4df72d434bef711  ruby-2.3.1.tar.gz' | shasum --check - && tar xzf ruby-2.3.1.tar.gz
-cd ruby-2.3.1
+curl --remote-name --progress https://cache.ruby-lang.org/pub/ruby/2.3/ruby-2.3.3.tar.gz
+echo 'a8db9ce7f9110320f33b8325200e3ecfbd2b534b ruby-2.3.3.tar.gz' | shasum -c - && tar xzf ruby-2.3.3.tar.gz
+cd ruby-2.3.3
 ./configure --disable-install-rdoc
 make
 sudo make install