fix: use plain code span for proto reference to avoid broken rustdoc link

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

quickwit/quickwit-parquet-engine/src/split/metadata.rs

@@ -176,9 +176,8 @@ pub struct MetricsSplitMetadata {
     pub num_merge_ops: u32,
 
     /// RowKeys (sort-key min/max boundaries) as serialized proto bytes
-    /// ([`sortschema::RowKeys`](../../quickwit-proto/protos/event_store_sortschema/
-    /// event_store_sortschema.proto)). None for pre-Phase-31 splits or splits without sort
-    /// schema.
+    /// (`sortschema::RowKeys` in `event_store_sortschema.proto`).
+    /// None for pre-Phase-31 splits or splits without sort schema.
     pub row_keys_proto: Option<Vec<u8>>,
 
     /// Per-column zonemap regex strings, keyed by column name.