Remove \n from puts() call

The newline is added by puts() so this led to double newlines.

app/main.c

@@ -20,7 +20,7 @@ static bool step_repl(void)
 	expression_t *expression;
 	const int used = parse_expression(&pool, buffer, len, &expression);
 	if (used != len) {
-		puts("Invalid expression\n");
+		puts("Invalid expression");
 		return true;
 	}