[DOCS] Documentation

[pst2154]

In the API Reference, you are missing a parentheses

`from collections import namedtuple

Transform = namedtuple('Transform', ['func', 'inv_func']
transform = Transform(func=numpy.exp, inv_func=numpy.log)

ht = HTSRegressor(transform=transform, ...)`

should be

`from collections import namedtuple

Transform = namedtuple('Transform', ['func', 'inv_func'])
transform = Transform(func=numpy.exp, inv_func=numpy.log)

ht = HTSRegressor(transform=transform, ...)`