doc: slight change in asciidoc config file to print vX.Y

doc/asciidoc.conf

@@ -26,8 +26,8 @@ template::[header-declarations]
 <refentrytitle>{mantitle}</refentrytitle>
 <manvolnum>{manvolnum}</manvolnum>
 <refmiscinfo class="source">libmodbus</refmiscinfo>
-<refmiscinfo class="version">{libmodbus_version}</refmiscinfo>
-<refmiscinfo class="manual">Libmodbus Manual</refmiscinfo>
+<refmiscinfo class="version">v{libmodbus_version}</refmiscinfo>
+<refmiscinfo class="manual">libmodbus Manual</refmiscinfo>
 </refmeta>
 <refnamediv>
   <refname>{manname}</refname>