Standardize heading format

src/01_getting_started/02_why_async.md

@@ -1,4 +1,4 @@
-## Why Async?
+# Why Async?
 
 We all love how Rust allows us to write fast, safe software. But why write
 asynchronous code?

src/01_getting_started/03_state_of_async_rust.md

@@ -1,4 +1,4 @@
-## The State of Asynchronous Rust
+# The State of Asynchronous Rust
 
 The asynchronous Rust ecosystem has undergone a lot of evolution over time,
 so it can be hard to know what tools to use, what libraries to invest in,

src/01_getting_started/05_http_server_example.md

@@ -2,7 +2,6 @@
 
 Let's use `async`/`.await` to build an echo server!
 
-
 To start, run `rustup update nightly` to make sure you've got the latest and
 greatest copy of Rust-- we're working with bleeding-edge features, so it's
 essential to stay up-to-date. Once you've done that, run

src/06_multiple_futures/01_chapter.md

@@ -5,6 +5,8 @@ the current task until a particular `Future` completes. However, real
 asynchronous applications often need to execute several different
 operations concurrently.
 
+# Executing Multiple Futures at a Time
+
 In this chapter, we'll cover some ways to execute multiple asynchronous
 operations at the same time:
 

src/06_multiple_futures/02_join.md

@@ -3,6 +3,8 @@
 The `futures::join` macro makes it possible to wait for multiple different
 futures to complete while executing them all concurrently.
 
+# `join!`
+
 When performing multiple asynchronous operations, it's tempting to simply
 `.await` them in a series: