Update README.md

Fix example referenced in https://github.com/twitter/secureheaders/pull/86/files

README.md

@@ -85,9 +85,11 @@ Or simply add it to application controller
 
 ```ruby
 ensure_security_headers(
-  :hsts => {:include_subdomains, :x_frame_options => false},
+  :hsts => {:include_subdomains => true, :max_age => 20.years.to_i}
+  :x_frame_options => false,
   :x_frame_options => 'DENY',
-  :csp => false)
+  :csp => false
+)
 ```
 
 ## Options for ensure\_security\_headers