Don't double indent function signature arguments

[DavisVaughan]

See https://github.com/tidyverse/style/issues/215

it should be this:

mcmc_derive_fun <- function(
  object,
  new_data = data_set(object),
  new_expr = NULL,
  ...
) {
  body
}

not

mcmc_derive_fun <- function(
    object,
    new_data = data_set(object),
    new_expr = NULL,
    ...
) {
  body
}