Fix clippy error

[oxarbitrage]

Clippy should pass but it is failing in the current main branch.

oxarbitrage@oxarbitrage-Lenovo-ideapad-320S-14IKB:~/redjubjub/messages_impl1/test/redjubjub$ cargo clippy --all-features --all-targets
    Checking tinytemplate v1.2.1
    Checking redjubjub v0.4.0 (/home/oxarbitrage/redjubjub/messages_impl1/test/redjubjub)
error[E0554]: `#![feature]` may not be used on the stable release channel
  --> src/lib.rs:12:34
   |
12 | #![cfg_attr(feature = "nightly", feature(external_doc))]
   |                                  ^^^^^^^^^^^^^^^^^^^^^

    Checking criterion v0.3.4
error: aborting due to previous error

For more information about this error, try `rustc --explain E0554`.
error: could not compile `redjubjub`

To learn more, run the command again with --verbose.
warning: build failed, waiting for other jobs to finish...
error: build failed
oxarbitrage@oxarbitrage-Lenovo-ideapad-320S-14IKB:~/redjubjub/messages_impl1/test/redjubjub$