DOCS: fix unicorn example (#102)

DOCS: fix unicorn example (#102)

Unicorn::Configurator complains with ArgumentError unless block parameters are passed

diff --git a/README.md b/README.md
index b32b48d..fb5a239 100644
--- a/README.md
+++ b/README.md
@@ -219,7 +219,7 @@ end
 For Unicorn / Passenger
 
 `‍``ruby
-after_fork do 
+after_fork do |_server, _worker|
   require 'prometheus_exporter/instrumentation'
   PrometheusExporter::Instrumentation::ActiveRecord.start(
     custom_labels: { type: "unicorn_worker" }, #optional params

GitHub sha: c433c81c