fix typos related to message retention (#317)

app/views/servers/advanced.html.haml

@@ -51,7 +51,7 @@
           .fieldSet__input
             = f.text_field :raw_message_retention_size, :class => 'input input--text'
             %p.fieldSet__text
-              The total amount of disk space (in bytes) to allow raw message data to use on the disk. Older messages will be deleted to keep
+              The total amount of disk space (in megabytes) to allow raw message data to use on the disk. Older messages will be deleted to keep
               the total usage below this amount.
 
       .fieldSetSubmit.fieldSetSubmit--wide.buttonSet