[trivial] Add missing space in plugin documentation.

src/search/tasks/default_value_axioms_task.cc

@@ -422,7 +422,7 @@ static plugins::TypedEnumPlugin<AxiomHandlingType> _enum_plugin({
          "Overapproximate negated axioms for all derived variables which "
          "have cyclic dependencies by setting an empty condition, "
          "indicating the default value can always be achieved for free. "
-         "For all other derived variables, the negated axioms are computed"
+         "For all other derived variables, the negated axioms are computed "
          "exactly. Note that this can potentially lead to a combinatorial "
          "explosion."}
     });