Documentation misses required packages for sysrepo installation

The installation instructions for libyang and sysrepo (in doc/guide/netconf.xml) are preceded by a list of prerequisite packages. That list appears to be OK for libyang but I found that I needed to install a further two: libev-dev and swig in order to build sysrepo on an Ubuntu 18.04 system. The list of packages should be updated.

Also, the list of packages is too long for one line (unless the browser is full screen): it should be split across two lines.