Clean up E0666 explanation

src/librustc_error_codes/error_codes/E0666.md

@@ -1,21 +1,25 @@
-`impl Trait` types cannot appear nested in the
-generic arguments of other `impl Trait` types.
+`impl Trait` types cannot appear nested in the generic arguments of other
+`impl Trait` types.
 
-Example of erroneous code:
+Erroneous code example:
 
 ```compile_fail,E0666
 trait MyGenericTrait<T> {}
 trait MyInnerTrait {}
 
-fn foo(bar: impl MyGenericTrait<impl MyInnerTrait>) {}
+fn foo(
+    bar: impl MyGenericTrait<impl MyInnerTrait>, // error!
+) {}
 ```
 
-Type parameters for `impl Trait` types must be
-explicitly defined as named generic parameters:
+Type parameters for `impl Trait` types must be explicitly defined as named
+generic parameters:
 
 ```
 trait MyGenericTrait<T> {}
 trait MyInnerTrait {}
 
-fn foo<T: MyInnerTrait>(bar: impl MyGenericTrait<T>) {}
+fn foo<T: MyInnerTrait>(
+    bar: impl MyGenericTrait<T>, // ok!
+) {}
 ```