subscriber: fix doc typo (#2548)

**This commit**
- tyoe -> type.

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

tracing-subscriber/src/fmt/writer.rs

@@ -16,7 +16,7 @@ use tracing_core::Metadata;
 /// This trait is already implemented for function pointers and
 /// immutably-borrowing closures that return an instance of [`io::Write`], such
 /// as [`io::stdout`] and [`io::stderr`]. Additionally, it is implemented for
-/// [`std::sync::Mutex`][mutex] when the tyoe inside the mutex implements
+/// [`std::sync::Mutex`][mutex] when the type inside the mutex implements
 /// [`io::Write`].
 ///
 /// # Examples