Replace hyphens (-) with em-dashes (—) (#1500)

The new Chromium class likes — like me! — to use dashes in the writing!
However, I believe it should use an em-dash instead of the hyphen.

Luckily this is easy: we have enabled “typographic quotes” in `mdbook`,
which also handles the conversion of `---` to `—` in the generated HTML.
So I normalized the single existing em-dash to a triple-dash to make it
more consistent (and hopefully make it easier for translators to
consistently enter these characters).

src/exercises/concurrency/dining-philosophers-async.md

@@ -1,4 +1,4 @@
-# Dining Philosophers - Async
+# Dining Philosophers --- Async
 
 See [dining philosophers](dining-philosophers.md) for a description of the
 problem.

src/exercises/concurrency/solutions-afternoon.md

@@ -1,6 +1,6 @@
 # Concurrency Afternoon Exercise
 
-## Dining Philosophers - Async
+## Dining Philosophers --- Async
 
 ([back to exercise](dining-philosophers-async.md))