[verification] Fix typo in error message.

src/verification.ml

@@ -108,7 +108,7 @@ let answer_saver limit config env config_prover dataset task =
         then filename
         else
           invalid_arg
-            (Fmt.str "File '%s' has an unsupported  extension" filename)
+            (Fmt.str "File '%s' has an unsupported extension" filename)
     in
     Dataset.interpret_predicate env ~on_model ~on_dataset task
   in