Rollup merge of rust-lang#72191 - GuillaumeGomez:cleanup-e0589, r=Dylan-DPC

Clean up E0589 explanation

r? @Dylan-DPC

Author: RalfJung

src/librustc_error_codes/error_codes/E0589.md

@@ -1,6 +1,8 @@
 The value of `N` that was specified for `repr(align(N))` was not a power
 of two, or was greater than 2^29.
 
+Erroneous code example:
+
 ```compile_fail,E0589
 #[repr(align(15))] // error: invalid `repr(align)` attribute: not a power of two
 enum Foo {