graph/encoding/dot: fix link to dot documentation
Add Olivier Wulveryck to A+C
// See the GraphViz DOT Guide and the DOT grammar for more information
// on using specific aspects of the DOT language:
//
-// DOT Guide: http://www.graphviz.org/Documentation/dotguide.pdf
+// DOT Guide: https://www.graphviz.org/pdf/dotguide.pdf
//
// DOT grammar: http://www.graphviz.org/doc/info/lang.html
//
