fix typo in introduction to scikit learn notebook

[Sohaib90]

This PR fixes a typo in the introduction-to-scikit-learn.ipynb

where in the section 1.2.2 Filling missing data and transforming categorical data with Scikit-Learn in the Note, variables is misspelled as vairables

However, for numerical vairables, you should only fill values on the test set that have been computed from the training set.

please check if it looks fine @aneagoie @mrdbourke