reformatted summary

src/SUMMARY.md

@@ -1,10 +1,12 @@
 # Summary
 
 [Introduction](./introduction.md)
+
 - [Some background information](./1_background_information.md)
 - [Trait objects and fat pointers](./2_trait_objects.md)
 - [Generators and Pin](./3_generators_pin.md)
 - [Pin](./4_pin.md)
 - [The main example](./6_future_example.md)
 - [Finished example (editable)](./8_finished_example.md)
+
 [Conclusion and exercises](./conclusion.md)