small docs bug

[Thomas Aarholt]

Describe the bug
The seasonality docs suggests the following code:

from functime.offsets import freq_to_sp
from functime.seasonality import add_fourier_terms

sp = freq_to_sp["1mo"][0]
X_new = X.pipe(add_fourier_terms(sp=sp, K=3)).collect()

Here, freq_to_sp["1mo"] should be freq_to_sp("1mo") (parenthesis, not square brackets)`