fixing typo

Author: Peter Schrammel

README.rdoc

@@ -31,7 +31,7 @@ Below is an example for configuring the middleware:
   run app
 
 revers_proxy_options set global options for all reverse proxies. Available options are:
-  * :preserve_proxy 
+  * :preserve_host
   * :username username for basic auth
   * :password password for basic auth
   * :matching is a global only option, if set to :first the first matched url will be requested (no ambigous error). Default: :all.