Improve mailbox generator usage docs

- Makes heading styles consistent with other generators
- Adds backticks to code snippets

actionmailbox/lib/rails/generators/mailbox/USAGE

@@ -1,11 +1,9 @@
 Description:
-============
     Generates a new mailbox class in app/mailboxes and invokes your template
     engine and test framework generators.
 
 Example:
-========
-    bin/rails generate mailbox inbox
+    `bin/rails generate mailbox inbox`
 
     creates an InboxMailbox class and test:
         Mailbox:  app/mailboxes/inbox_mailbox.rb