add trailing underscore in ConstantStepProbeSizeRaiser
add trailing underscore in ConstantStepProbeSizeRaiser

Summary: just a nit

Reviewed By: xttjsn

Differential Revision: D23846840

fbshipit-source-id: b9c646255ff9db4344f7f486ae9f02c7ab322641

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst