doc: fixup NAT-PMP help doc

This always defaults to false, since we removed the compile time options
to set it otherwise.

src/init.cpp

@@ -524,7 +524,7 @@ void SetupServerArgs(ArgsManager& argsman)
     hidden_args.emplace_back("-upnp");
 #endif
 #ifdef USE_NATPMP
-    argsman.AddArg("-natpmp", strprintf("Use NAT-PMP to map the listening port (default: %s)", DEFAULT_NATPMP ? "1 when listening and no -proxy" : "0"), ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
+    argsman.AddArg("-natpmp", strprintf("Use NAT-PMP to map the listening port (default: %u)", DEFAULT_NATPMP), ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
 #else
     hidden_args.emplace_back("-natpmp");
 #endif // USE_NATPMP