Update error code documentation.

Author: cjgillot

compiler/rustc_error_codes/src/error_codes/E0760.md

@@ -1,4 +1,4 @@
-`async fn`/`impl trait` return type cannot contain a projection
+`impl trait` return type cannot contain a projection
 or `Self` that references lifetimes from a parent scope.
 
 Erroneous code example:
@@ -7,7 +7,7 @@ Erroneous code example:
 struct S<'a>(&'a i32);
 
 impl<'a> S<'a> {
-    async fn new(i: &'a i32) -> Self {
+    fn new(i: &'a i32) -> impl Into<Self> {
         S(&22)
     }
 }
@@ -19,7 +19,7 @@ To fix this error we need to spell out `Self` to `S<'a>`:
 struct S<'a>(&'a i32);
 
 impl<'a> S<'a> {
-    async fn new(i: &'a i32) -> S<'a> {
+    fn new(i: &'a i32) -> impl Into<S<'a>> {
         S(&22)
     }
 }