A decrease in electronegativity happens when you go down group 17. This is due to the fact that as you go down the group, the atomic radius goes up which means the first ionization energy decreases since shielding goes up. All of those facts above shows that as you go down the group the atom looses the ability to draw electrons to itself which means that the electronegativity is going down the further down the group you go.
I hope this helps. Let me know in the comments if anything is unclear or if you want me to add any further explanation.
