docs: add missing import to RowNumber example

[peter-gy]

Description of changes

The existing example was not self-contained. Upon execution, it raised an error as dt.int64 could not be resolved.

>>> import ibis
>>> t = ibis.table([("values", dt.int64)])  # NameError: name 'dt' is not defined
>>> w = ibis.window(order_by=t.values)
>>> row_num = ibis.row_number().over(w)
>>> result = t[t.values, row_num.name("row_num")]

This PR adds the missing import ibis.expr.datatypes as dt statement.

[lostmygithubaccount]

thanks!