docs: add a parenthesis (#1597)

## Motivation
Looks like doc is missing a parenthesis


## Solution
Add a parenthesis

tracing/src/lib.rs

@@ -173,7 +173,7 @@
 //! ```
 //!
 //! For functions which don't have built-in tracing support and can't have
-//! the `#[instrument]` attribute applied (such as from an external crate,
+//! the `#[instrument]` attribute applied (such as from an external crate),
 //! the [`Span` struct][`Span`] has a [`in_scope()` method][`in_scope`]
 //! which can be used to easily wrap synchonous code in a span.
 //!