PG 10 release notes: update PL/Tcl functions item

Update attribution of PL/Tcl functions item from Jim Nasby to Karl
Lehenbauer.

Reported-by: Jim Nasby

Discussion: https://postgr.es/m/ed42f3d6-4251-dabc-747f-1ff936763b2b@nasby.net

Backpatch-through: 10