Fix em dashes

Author: Max Hausch

src/01_getting_started/02_why_async.md

@@ -12,7 +12,7 @@ different threads, like this:
 {{#include ../../examples/01_02_why_async/src/lib.rs:get_two_sites}}
 ```
 
-This works fine for many applications-- after all, threads were designed
+This works fine for many applications—after all, threads were designed
 to do just this: run multiple different tasks at once. However, they also
 come with some limitations. There's a lot of overhead involved in the
 process of switching between different threads and sharing data between
@@ -29,7 +29,7 @@ once without creating multiple threads:
 Overall, asynchronous applications have the potential to be much faster and
 use fewer resources than a corresponding threaded implementation. However,
 there is a cost. Threads are natively supported by the operating system,
-and using them doesn't require any special programming model-- any function
+and using them doesn't require any special programming model—any function
 can create a thread, and calling a function that uses threads is usually
 just as easy as calling any normal function. However, asynchronous functions
 require special support from the language or libraries.

src/01_getting_started/04_async_await_primer.md

@@ -48,7 +48,7 @@ individually:
 {{#include ../../examples/01_04_async_await_primer/src/lib.rs:block_on_each}}
 ```
 
-However, we're not giving the best performance possible this way-- we're
+However, we're not giving the best performance possible this way—we're
 only ever doing one thing at once! Clearly we have to learn the song before
 we can sing it, but it's possible to dance at the same time as learning and
 singing the song. To do this, we can create two separate `async fn` which

src/07_workarounds/05_recursion.md

@@ -33,7 +33,7 @@ enum Recursive {
 }
 ```
 
-This won't work-- we've created an infinitely-sized type!
+This won't work—we've created an infinitely-sized type!
 The compiler will complain:
 
 ```