lubridate: specify `months()` as base function

[mine-cetinkaya-rundel]

In the lubridate cheatsheet (last updated 2021-07), specifically the Periods section on the second page, there is a reference to a months(x) function. I initially thought that the months function was from the lubridate package, only to realize that it is from the base package. It would be helpful to add a note or a specific reference (such as base::months(x)) to clarify where this function comes from. An alternative possibility could also be to remove months() from the list of functions in that section.

(via email from Anna Vasylytsya -- [email protected])