Update char.html - remove link to list of named chars

There are so many named characters on that link that I think Graphviz probably doesn't support them all.

From https://forum.graphviz.org/t/bi-color-nodes-and-special-characters/252/6:

""" The references on this page also work: https://www.graphviz.org/doc/char.html (That page links to two pages on w3.org: the first has the same information, but the second (“an official version of this reference”) is not the same at all and not useful for GV.) """