[boykov-kolmogorov] mismatch between doc/figure on colors

[eolivi]

In the lib documentation, https://www.boost.org/doc/libs/1_70_0/libs/graph/doc/boykov_kolmogorov_max_flow.html the figure shows a white Source node and a black Sink node, whether it is previously written that:

the source and the sink are colored (source==black, sink==white)

This is confusing. Either change the picture or the code/doc.

IMO the source should be white and not black (i.e change the code and doc), but it would be too complicated to keep backward compatibility...