changes docker image name from python to ruby

doc/examples/test-and-deploy-ruby-application-to-heroku.md

@@ -58,10 +58,10 @@ gitlab-ci-multi-runner register \
   --registration-token "PROJECT_REGISTRATION_TOKEN" \
   --description "ruby-2.1" \
   --executor "docker" \
-  --docker-image python:2.1 \
+  --docker-image ruby:2.1 \
   --docker-postgres latest
 ```
 With the command above, you create a runner that uses [ruby:2.1](https://registry.hub.docker.com/u/library/ruby/) image and uses [postgres](https://registry.hub.docker.com/u/library/postgres/) database.
-To access PostgreSQL database you need to connect to `host: postgres` as user `postgres` without password.
+To access PostgreSQL database you need to connect to `host: postgres` as user `postgres` without password.
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