What celeberty did Fred Silverman name Scooby Doo after?

Fred Silverman did not name Scooby-Doo. The character was created by Joe Ruby and Ken Spears, and they chose the name Scooby-Doo because they liked the sound of it. It's a playful combination of "Scooby" (a shortened form of "Scoob") and "Doo" (a common ending for pet names).

There's no evidence of a connection to any particular celebrity.