Dead links for documentation

[Jacob Holloway]

I was reading through the source code about attributes and found some URLs to GraphViz's webpage that are now dead. It looks like they have migrated. I don't have time right now to do a PR, but here's the new url:

/* Old */
// Damping http://www.graphviz.org/content/attrs#dDamping

/* New */
// Damping http://graphviz.gitlab.io/_pages/doc/info/attrs.html#d:Damping

Hope that helps a little bit. :) Thanks for the work you've already done! Super helpful!

[Walter Schulze]

Thank you so much for reporting :)

[Walter Schulze]

fixed 16ed621