improve readme markup for send_file documentation

README.rdoc

@@ -857,14 +857,21 @@ It also take a couple of options:
 
 The options are:
 
-* <tt>:filename</tt> - file name in response, defaults to the real file name.
-* <tt>:last_modified</tt> - value for Last-Modified header, defaults to the
-  file's mtime.
-* <tt>:type</tt> - content type to use, guessed from the file extension if
-  missing.
-* <tt>:disposition</tt> - used for Content-Disposition, possible values: +nil+
-  (default), <tt>:attachement</tt> and <tt>:inline</tt>
-* <tt>:length</tt> - Content-Length header, defaults to file size.
+[filename]
+  file name in response, defaults to the real file name.
+
+[last_modified]
+  value for Last-Modified header, defaults to the file's mtime.
+
+[type]
+  content type to use, guessed from the file extension if missing.
+
+[disposition]
+  used for Content-Disposition, possible values: +nil+ (default),
+  <tt>:attachement</tt> and <tt>:inline</tt>
+
+[length]
+  Content-Length header, defaults to file size.
 
 If supported by the Rack handler, other means than streaming from the Ruby
 process will be used. If you use this helper method, Sinatra will automatically