chore(nix-compat/nar/wire): space Tag::Buf docs properly

Change-Id: If15b14b5328b9e7f85ea3fa7644d327dfb50e1a6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11523
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/nix-compat/src/nar/wire/tag.rs

@@ -10,6 +10,7 @@ pub trait Tag: Sized {
     const MIN: usize;
 
     /// Minimal suitably sized buffer for reading the wire representation
+    ///
     /// HACK: This is a workaround for const generics limitations.
     type Buf: AsMut<[u8]> + Send;