Fix typo in doc/build-unix.md regarding building without UPnP

Author: runeksvendsen

doc/build-unix.md

@@ -28,7 +28,7 @@ Dependencies
 http://miniupnp.tuxfamily.org/files/).  UPnP support is compiled in and
 turned off by default.  See the configure options for upnp behavior desired:
 
-	--with-miniupnpc         No UPnP support miniupnp not required
+	--without-miniupnpc      No UPnP support miniupnp not required
 	--disable-upnp-default   (the default) UPnP support turned off by default at runtime
 	--enable-upnp-default    UPnP support turned on by default at runtime