[eigen] Just a tiny typo en Dox docs

[Jose Luis Blanco <joseluisblancoc@xxxxxxxxx>]

Please, anyone with write access to the repository, check this:

In:  http://eigen.tuxfamily.org/dox-devel/TopicStlContainers.html

This line:
std::map<int, Eigen::Vector4f, std::less<int>,
Eigen::aligned_allocator<Eigen::Vector4f> >

I think it should be:
std::map<int, Eigen::Vector4f, std::less<int>,
Eigen::aligned_allocator<std::pair<const int, Eigen::Vector4f> > >

Best,
JL