fix maxVtx definition in fillDescription method (needed for confDB)

w.r.t. PR 3972
this PR has the parameter maxVtx_ as unsinged int

so, PR 3972 should be superseded by this one
I'll close PR 3972 as soon as you will give the green light to this one

@perrotta @bocci @cerati @Martin-Grunewald @rovere