Remove section on return type errors

Author: lbernick

src/07_workarounds/03_err_in_async_blocks.md renamed to src/07_workarounds/02_err_in_async_blocks.md

@@ -20,11 +20,10 @@
   - [TODO: Cancellation and Timeouts](404.md)
   - [TODO: `FuturesUnordered`](404.md)
 - [Workarounds to Know and Love](07_workarounds/01_chapter.md)
-  - [Return Type Errors](07_workarounds/02_return_type.md)
-  - [`?` in `async` Blocks](07_workarounds/03_err_in_async_blocks.md)
-  - [`Send` Approximation](07_workarounds/04_send_approximation.md)
-  - [Recursion](07_workarounds/05_recursion.md)
-  - [`async` in Traits](07_workarounds/06_async_in_traits.md)
+  - [`?` in `async` Blocks](07_workarounds/02_err_in_async_blocks.md)
+  - [`Send` Approximation](07_workarounds/03_send_approximation.md)
+  - [Recursion](07_workarounds/04_recursion.md)
+  - [`async` in Traits](07_workarounds/05_async_in_traits.md)
 - [Final Project: HTTP Server](08_example/00_intro.md)
   - [Running Asynchronous Code](08_example/01_running_async_code.md)
   - [Handling Connections Concurrently](08_example/02_handling_connections_concurrently.md)