link to async context

Author: nikomatsakis

src/expressions/await-expr.md

@@ -4,7 +4,7 @@
 > _AwaitExpression_ :\
 >    [_Expression_] `.` `await`
 
-Await expressions are legal only within `async` contexts, like an
+Await expressions are legal only within an [async context], like an
 [`async fn`] or an [`async` block]. They operate on a [future]. Their effect
 is to suspend the current computation until the given future is ready
 to produce a value.