[BGL] small documentation error

[gast128@…]

In the chapter about 'Bundled Properties', an example is made with Highway. It has a member 'double miles'.

Later on: "The type of the returned property map is property_map<Map, int Highway::*>::type..."

However this gives a compile error. Probably meant 'property_map<Map, double Highway::*>'

[Jeremiah Willcock]

(In [70875]) Fixed types in bundled property docs; fixes #5410