try to fix -Wchanges-meaning

Author: HuaHuaY

src/iceberg/table_metadata.h

@@ -122,7 +122,7 @@ struct ICEBERG_EXPORT TableMetadata {
   int64_t next_row_id;
 
   /// \brief Get the current schema, return NotFoundError if not found
-  Result<std::shared_ptr<Schema>> Schema() const;
+  Result<std::shared_ptr<iceberg::Schema>> Schema() const;
   /// \brief Get the current schema by ID, return NotFoundError if not found
   Result<std::shared_ptr<iceberg::Schema>> SchemaById(
       const std::optional<int32_t>& schema_id) const;