sym: tweak error message: Nary -> Varargs

Author: oscarbenjamin

src/protosym/core/sym.py

@@ -562,7 +562,7 @@ def __setitem__(  # noqa: C901
         if isinstance(call.op, PyOpN):
             (callarg,) = call.args
             if pattern.args != (star(callarg),):
-                raise BadRuleError("Nary function needs a star-rule.")
+                raise BadRuleError("varargs function needs a star-rule.")
         elif pattern.args != call.args:
             raise BadRuleError("Pattern and rule signatures do not match.")