docs: Fix proxy setting sample code

[nicklegr]

Current code doesn't work. I got following error:

/usr/src/app/vendor/bundle/ruby/2.2.0/gems/http-3.0.0/lib/http/chainable.rb:164:in `via': invalid HTTP proxy: {} (HTTP::RequestError)
 from /usr/src/app/vendor/bundle/ruby/2.2.0/gems/twitter-6.2.0/lib/twitter/rest/request.rb:120:in `http_client'
 from /usr/src/app/vendor/bundle/ruby/2.2.0/gems/twitter-6.2.0/lib/twitter/rest/request.rb:36:in `perform'
 from /usr/src/app/vendor/bundle/ruby/2.2.0/gems/twitter-6.2.0/lib/twitter/search_results.rb:26:in `initialize'
 from /usr/src/app/vendor/bundle/ruby/2.2.0/gems/twitter-6.2.0/lib/twitter/rest/search.rb:33:in `new'
 from /usr/src/app/vendor/bundle/ruby/2.2.0/gems/twitter-6.2.0/lib/twitter/rest/search.rb:33:in `search'
 from test.rb:10:in `<main>'

See:

• https://github.com/sferik/twitter/blob/master/lib/twitter/rest/request.rb#L129
• https://github.com/httprb/http/blob/v3.0.0/lib/http/chainable.rb#L164