fix: use store_true on argparse in nlp example

[monologg]

In example, it makes error when using the commands below.

python ./nlp_example.py --cpu
python ./nlp_example.py --fp16

So fixed the argparse option as --store_true, which is also same in cv_example.py

accelerate/examples/cv_example.py

Lines 189 to 190 in c5c73e0

	parser.add_argument("--fp16", action="store_true", help="If passed, will use FP16 training.")
	parser.add_argument("--cpu", action="store_true", help="If passed, will train on the CPU.")

[sgugger]

Thanks for fixing!