docs: the code will raise error if is executed in chapter_preliminaries's pandas.md

[Halory-Ito]

inputs, outputs = data.iloc[:, 0:2], data.iloc[:, 2]

inputs = inputs.apply(pd.to_numeric, errors='coerce') 
# without this line, will raise TypeError: Cannot perform reduction 'mean' with string dtype


inputs = inputs.fillna(inputs.mean())
print(inputs)

here is the error screenshot: