xhttp_rpc: documentation correction

- The documentation in the README file had the
  wrong module name in the modparam in the
  example for the xhttp_rpc_buf_size parameter.
  Corrected this in section 4.2.

(cherry picked from commit c9229d6)

src/modules/xhttp_rpc/doc/xhttp_rpc_admin.xml

@@ -125,7 +125,7 @@ modparam("xhttp_rpc", "xhttp_rpc_root", "http_rpc")
 		<title>Set <varname>xhttp_rpc_buf_size</varname> parameter</title>
 		<programlisting format="linespecific">
 ...
-modparam("xhttp", "xhttp_rpc_buf_size", 1024)
+modparam("xhttp_rpc", "xhttp_rpc_buf_size", 1024)
 ...
 </programlisting>
 		</example>