tag examples with edition2018

Author: nikomatsakis

src/items/functions.md

@@ -197,7 +197,7 @@ Exhaustive list of permitted structures in const functions:
 Functions may be qualified as async, and this can also be combined with the
 `unsafe` qualifier:
 
-```rust
+```rust,edition2018
 async fn regular_example() { }
 async unsafe fn unsafe_example() { }
 ```
@@ -209,7 +209,7 @@ execute the function's body.
 An async function is roughly equivalent to a function
 that returns [`impl Future`] and with an [`async move` block] as its body:
 
-```rust
+```rust,edition2018
 // Source
 async fn example(x: &str) -> usize {
     x.len()