shrpx: Update --backend-http-proxy-uri usage

src/shrpx.cc

@@ -476,8 +476,9 @@ void print_help(std::ostream& out)
       << get_config()->downstream_idle_read_timeout.tv_sec << "\n"
       << "    --backend-http-proxy-uri=<URI>\n"
       << "                       Specify proxy URI in the form\n"
-      << "                       http://[USER:PASS]PROXY:PORT. USER and PASS\n"
-      << "                       are optional and if they exist they must be\n"
+      << "                       http://[<USER>:<PASS>@]<PROXY>:<PORT>. If\n"
+      << "                       a proxy requires authentication, specify\n"
+      << "                       <USER> and <PASS>. Note that they must be\n"
       << "                       properly percent-encoded. This proxy is used\n"
       << "                       when the backend connection is SPDY. First,\n"
       << "                       make a CONNECT request to the proxy and\n"