fix typos related to message retention (#317)

app/views/servers/retention.html.haml

@@ -1,5 +1,5 @@
 - page_title << @server.name
-- page_title << "Message Rentention"
+- page_title << "Message Retention"
 = render 'sidebar', :active_server => @server
 = render 'header', :active_nav => :settings
 = render 'settings_header', :active_nav => :retention