Doc: Unicorn minimum worker_processes is 2

A value of 1 cause http push fail (issue #6978 on GitHub)

config/unicorn.rb.example

@@ -15,6 +15,7 @@
 
 # Use at least one worker per core if you're on a dedicated server,
 # more will usually help for _short_ waits on databases/caches.
+# The minimum is 2
 worker_processes 2
 
 # Since Unicorn is never exposed to outside clients, it does not need to