Corrected description of DGraph sinks()
Corrected description of DGraph sinks()

Was incorrectly describing a sink as a node with no
in edges. Changed the description to say that a sink
is a node with no out edges

cpettitt/graphlib,kdawes/graphlib,cpettitt/graphlib,gardner/graphlib,dagrejs/graphlib,leMaik/graphlib,leMaik/graphlib,dagrejs/graphlib,gardner/graphlib,kdawes/graphlib,dagrejs/graphlib