[pulse][minor] resolve mysterious 'a in .mli

Summary:
Even though it's unused in the implementation this argument is supposed
to be `Typ.t` so spell it out.

Reviewed By: skcho

Differential Revision: D23729433

fbshipit-source-id: d05548f42

infer/src/pulse/PulseOperations.mli

@@ -138,7 +138,7 @@ val call :
 val unknown_call :
      Location.t
   -> CallEvent.t
-  -> ret:Ident.t * 'a
+  -> ret:Ident.t * Typ.t
   -> actuals:((AbstractValue.t * ValueHistory.t) * Typ.t) list
   -> formals_opt:(Pvar.t * Typ.t) list option
   -> t