chore: fix unused_doc_comments warning (#1309)

Signed-off-by: Ruihang Xia <[email protected]>

Author: waynexia

opentelemetry-stackdriver/src/lib.rs

@@ -328,8 +328,8 @@ where
                 ),
                 display_name: Some(to_truncate(span.name.into_owned())),
                 span_id: hex::encode(span.span_context.span_id().to_bytes()),
-                /// From the API docs: If this is a root span,
-                /// then this field must be empty.
+                // From the API docs: If this is a root span,
+                // then this field must be empty.
                 parent_span_id: match span.parent_span_id {
                     SpanId::INVALID => "".to_owned(),
                     _ => hex::encode(span.parent_span_id.to_bytes()),