Fix assert in model.py

[medvedev1088]

Found this bug when testing on a small dataset it fails when the number of items in the dataset is 3, the number of users is 10, the user rated 2, and I try to get 2 recommendations.

The assert is supposed to check that the number of items in the dataset is greater than the number of requested recommendations plus the number of items a user already rated. The reason I changed row_factor with col_factor is col_factor.shape[0] is the number of items in the dataset.

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[medvedev1088]

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