docs: remove incorrect total_difficulty mention from process_iter (#20234)

Co-authored-by: Alexey Shekhirin <[email protected]>

Author: forkfury

crates/era-utils/src/history.rs

@@ -252,7 +252,7 @@ where
 
 /// Extracts block headers and bodies from `iter` and appends them using `writer` and `provider`.
 ///
-/// Adds on to `total_difficulty` and collects hash to height using `hash_collector`.
+/// Collects hash to height using `hash_collector`.
 ///
 /// Skips all blocks below the [`start_bound`] of `block_numbers` and stops when reaching past the
 /// [`end_bound`] or the end of the file.