#4853 CG: graphviz instructions contain old build commands

[Federico Bruni]

As it came out on issue [#4809], the CG instructions on graphviz contain wrong information about the build.
The two occurrences of make -C lily clean && make -C lily should be replaced with make clean && make.

I would also suggest that graphviz is present in most Linux ditributions.
BTW, the link in the doc has two dots:

http://www.graphviz.org/Download..php

but I'd rather link to the home page instead.

[Phil Holmes]

Trivial patch to correct make syntax tested with make doc and then pushed to staging:

commit 6cceacc45f484426b17798cf15d79a48ec12141d