README: move the ronn command to the optional dependencies description section.

README.md

@@ -40,12 +40,12 @@ Optionally, you need:
 
 1.  the pulseaudio development library to enable pulseaudio output;
 2.  the portaudio development library to enable portaudio output;
-3.  the sonic development library to enable sonic audio speed up support.
+3.  the sonic development library to enable sonic audio speed up support;
+4.  the `ronn` man-page markdown processor to build the man pages.
 
 To build the documentation, you need:
 
 1.  the `kramdown` markdown processor.
-2.  the `ronn` man-page markdown processor to build the man pages.
 
 ### Debian