README.md: Tweak installation instructions
README.md: Tweak installation instructions

SILE's required libraries don't need to be installed with luarocks;
installing them through my system's package manager worked just fine
for me.

While here, mention what the "PDF creation library" is called and link
to it, and fix a typo.

neofob/sile,simoncozens/sile,alerque/sile,alerque/sile,alerque/sile,neofob/sile,simoncozens/sile,neofob/sile,alerque/sile,simoncozens/sile,neofob/sile,simoncozens/sile