Typo in cumulative function docs

[DavisVaughan]

Good first issue for TDD if you've never done a PR before!

dplyr/R/funs.R

Line 52 in 0005f67

#' Cumulativate versions of any, all, and mean

Cumulativate -> Cumulative

Don't forget to redocument!

[kyleGrealis]

This was closed by #7068