core: fix missing dyn in Visit docs example (#1595)

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

tracing-core/src/field.rs

@@ -118,7 +118,7 @@ pub struct Iter {
 /// }
 ///
 /// impl<'a> Visit for StringVisitor<'a> {
-///     fn record_debug(&mut self, field: &Field, value: &fmt::Debug) {
+///     fn record_debug(&mut self, field: &Field, value: &dyn fmt::Debug) {
 ///         write!(self.string, "{} = {:?}; ", field.name(), value).unwrap();
 ///     }
 /// }