Indeed wikipedia has columns “text” and “title” while bookcorpus only has “text”.
You can concatenate them by removing the “title” column from wikipedia:
from datasets import load_dataset, concatenate_datasets

wiki = load_dataset("wikipedia", "20200501.en", split="train")
bookcorpus = load_dataset("bookcorpus", split="train")
print(wiki.column_names, bookcorpus.column_names)
# ['title', 'text'] ['text']

wiki.remove_columns_("title")
bert_dataset = concatenate_datasets([wiki, bookcorpus])