docs: fix backporting error in attributes (tokio-rs#2780)

There was an error when backporting tokio-rs#1378 (here: tokio-rs#1418) and a trailing
dot (.) was forgotten (which was breaking the link). Fixed also the link to
`std::fmt::Debug`.

Author: kaffarell

tracing-attributes/src/lib.rs

@@ -94,7 +94,7 @@ mod expand;
 /// By default, all arguments to the function are included as fields on the
 /// span. Arguments that are `tracing` [primitive types] implementing the
 /// [`Value` trait] will be recorded as fields of that type. Types which do
-/// not implement `Value` will be recorded using [`std::fmt::Debug`].
+/// not implement `Value` will be recorded using [`fmt::Debug`].
 ///
 /// [primitive types]: https://docs.rs/tracing/latest/tracing/field/trait.Value.html#foreign-impls
 /// [`Value` trait]: https://docs.rs/tracing/latest/tracing/field/trait.Value.html