Rename custom dataset function to comply with documentation

examples/custom_dataset.py

@@ -7,7 +7,7 @@ import datasets
 
 from llama_recipes.datasets.utils import Concatenator
 
-def get_preprocessed_samsum(dataset_config, tokenizer, split):
+def get_custom_dataset(dataset_config, tokenizer, split):
     dataset = datasets.load_dataset("samsum", split=split)
 
     prompt = (