chunk_size should be instead of block_size

`chunk_size` should be instead of `block_size` (`block_size` was never mentioned before)

chapters/en/chapter7/3.mdx

@@ -347,7 +347,7 @@ print(f"'>>> Concatenated reviews length: {total_length}'")
 '>>> Concatenated reviews length: 951'
 ```
 
-Great, the total length checks out -- so now let's split the concatenated reviews into chunks of the size given by `block_size`. To do so, we iterate over the features in `concatenated_examples` and use a list comprehension to create slices of each feature. The result is a dictionary of chunks for each feature:
+Great, the total length checks out -- so now let's split the concatenated reviews into chunks of the size given by `chunk_size`. To do so, we iterate over the features in `concatenated_examples` and use a list comprehension to create slices of each feature. The result is a dictionary of chunks for each feature:
 
 ```python
 chunks = {