Docs for updating inference function show wrong inferred Logical Type

[Here](https://woodwork.alteryx.com/en/latest/guides/custom_types_and_type_inference.html#Defining-Custom-Inference-Functions).

In this section, we have `Also note that the not_upc column was identified as Categorical.`. When you look at the schema results a few lines down, `not_upc` is actually inferred as Unknown. This may not be a doc issue but a problem with the `Categorical` inference function.