[ELECTRA/TF2] Option To Allow scripts/run_pretraining.sh To Use "wiki_only"

[psharpe99]

Related to ELECTRA/TF2

Is your feature request related to a problem? Please describe.
The README shows that the datasets can be created from wiki-only:
/workspace/electra/data/create_datasets_from_start.sh wiki_books
but when you then continue to pretrain using the README instruction
bash scripts/run_pretraining.sh
it complains about the file/directory not existing.
Looking at the run_pretraining.sh script, it has
DATASET_P1="tfrecord_lower_case_1_seq_len_128_random_seed_12345/books_wiki_en_corpus/train/pretrain_data*" # change this for other datasets
DATASET_P2="tfrecord_lower_case_1_seq_len_512_random_seed_12345/books_wiki_en_corpus/train/pretrain_data*" # change this for other datasets
which are preset to the books_wiki directory, with the comment that these need to be (manually) "changed" for other datasets (e.g. wiki-only)
Changing these manually to the 'wikicorpus_en' directory allowed the pretraining to succeed, but the script ideally shouldn't need editing.

Describe the solution you'd like
It should be a simple change to include a command-line option to the run_pretraining script for "wiki-only" .

Describe alternatives you've considered
Alternatively, it should be documented in the README that this script file needs to be editted if running only from wiki data.

Additional context
none