Incorrect use of `deprecated` attribute

[estebank]

rust-lang/rust#56896 is making the correct use of deprecated mandatory in rustc. We can't apply this as a hard error due to the following use in frunk_core:

[INFO] [stderr]    Compiling frunk_core v0.2.2
[INFO] [stderr] error: expected meta item sequence
[INFO] [stderr]    --> /opt/crater/cargo-home/registry/src/github.com-1ecc6299db9ec823/frunk_core-0.2.2/src/labelled.rs:223:5
[INFO] [stderr]     |
[INFO] [stderr] 223 |     #[deprecated = "obsolete, transform_from instead"]
[INFO] [stderr]     |     ^^-----------------------------------------------^
[INFO] [stderr]     |       |
[INFO] [stderr]     |       help: use the `note` key: `deprecated(note = "obsolete, transform_from instead")`