Fix compile error

Before this commit, Poise compilation would fail with this error:

```
error[E0432]: unresolved import `crate::serenity`
  --> src/builtins/mod.rs:20:13
   |
20 | use crate::{serenity::CreateAllowedMentions, serenity_prelude as serenity, CreateReply};
   |             ^^^^^^^^
   |             |
   |             unresolved import
   |             help: a similar path exists: `crate::prefix_argument::key_value_args::serenity`

For more information about this error, try `rustc --explain E0432`.
error: could not compile `poise` (lib) due to 1 previous error
```

src/builtins/mod.rs

@@ -17,7 +17,8 @@ mod paginate;
 #[cfg(any(feature = "chrono", feature = "time"))]
 pub use paginate::*;
 
-use crate::{serenity::CreateAllowedMentions, serenity_prelude as serenity, CreateReply};
+use crate::{serenity_prelude as serenity, CreateReply};
+use serenity::CreateAllowedMentions;
 
 /// An error handler that logs errors either via the [`tracing`] crate or via a Discord message. Set
 /// up a logger (e.g. `env_logger::init()`) or a tracing subscriber