revise mbpp prompt

[bzantium]

I talked with @hjlee1371 who firstly commited mbpp task and he confirmed new prompt is right.
resolved: #2644

[bzantium]

When I tested original code with Qwen2.5-32B (this is why I commited this PR):

Tasks	Version	Filter	n-shot	Metric		Value		Stderr
mbpp	1	none	3	pass_at_1	↑	0	±	0

with revised code:

Tasks	Version	Filter	n-shot	Metric		Value		Stderr
mbpp	1	n=1	3	pass_at_1	↑	0.732	±	0.0197

I also tested other models, some are okay with original code but all work well with revised code.
one test failed because of absence of os.environ["HF_ALLOW_CODE_EVAL"] = "1"

to: @baberabb

[baberabb]

LSTM! Great work!