fix import error message for sklearn

[wubing101]

for sklearn, if needing install it, the ImportError should write "pip install scikit-learn" instead of "pip install sklearn"
now:
File "/home/ubuntu/wubing/evaluate/src/evaluate/loading.py", line 265, in _download_additional_modules
raise ImportError(
ImportError: To be able to use evaluate-metric/accuracy, you need to install the following dependencies['sklearn'] using 'pip install scikit-learn' for instance'

before:
File "/home/ubuntu/anaconda3/envs/swift_llm/lib/python3.9/site-packages/evaluate/loading.py", line 265, in _download_additional_modules
raise ImportError(
ImportError: To be able to use evaluate-metric/accuracy, you need to install the following dependencies['scikit-learn'] using 'pip install sklearn' for instance'