rgw_cksum: fix ReadOp comment

parts_count now returns a value for parts_count whenever the
target object is a multipart upload.  this has no additional
overhead, since this can be read off the manifest

Signed-off-by: Matt Benjamin <[email protected]>

Author: mattbenjamin

src/rgw/rgw_sal.h

@@ -1049,8 +1049,8 @@ class Object {
 
         /// If non-null, read data/attributes from the given multipart part.
         int* part_num{nullptr};
-        /// If part_num is specified and the object is multipart, the total
-        /// number of multipart parts is assigned to this output parameter.
+        /// If the object is multipart, the total number of multipart
+        /// parts is assigned to this output parameter.
         std::optional<int> parts_count;
       } params;