Fix problems with RD files

[dincerti]

From the CRAN checks (https://cran.r-project.org/web/checks/check_results_hesim.html).

For example, the items in itemize shouldn't hae braces. create_params.Rd should be derived in Roxygen as:

#' \itemize{
#' \item `create_params.lm` -> [`params_lm`]
#' \item `create_params.multinom` -> [`params_mlogit`]
#' \item `create_params.multinom_list` ->  [`params_mlogit_list`]
#' \item `create_params.flexsurvreg` -> [`params_surv`]
#' \item `create_params.flexsurvreg_list` -> [`params_surv_list`]
#' \item `create_params.partsurvfit` ->  [`params_surv_list`]
#' }