docs: update "len" to "length"

docs/_tutorials/tutorial_3_parse_ble_tlv_responses/tutorial.md

@@ -264,7 +264,7 @@ The snippets of code included in this section are taken from the `notification h
 
 ```python
 # Parse first 3 bytes
-len = data[0]
+length = data[0]
 command_id = data[1]
 status = data[2]
 ```