Update src/items/functions.md

Co-Authored-By: Mazdak Farrokhzad <[email protected]>

Author: nikomatsakis

src/items/functions.md

@@ -258,7 +258,7 @@ returns a future. This future is just an ordinary future and thus an
 ```rust,edition2018
 // Returns a future that, when awaited, dereferences `x`.
 //
-// Unsafe condition: `x` must be safe to dereference until
+// Soundness condition: `x` must be safe to dereference until
 // the resulting future is complete.
 async unsafe fn unsafe_example(x: *const i32) -> i32 {
   *x