Replace surveys$id[i] with "SV_xxxxxxx1" to close #341

[JeffreyRStevens]

This PR addresses #341 by replacing surveys$id[i] with "SV_xxxxxxx1" in all documentation. This seems like a more robust way to name the survey ID, since deleting surveys could change a particular survey's position in the list of surveys. Therefore, I made the replacement in all instances in the README, vignette, and examples but describe how to use surveys$id[i] in the vignette. But I'm happy to alter this, if desired.

[juliasilge]

Thank you so much for this PR, @JeffreyRStevens!

I don't want to change all these use cases like this based on other feedback and use cases for our docs, but we can definitely add it to the docs for fetch_survey() (not replace the current approach). For example, we could edit those exmaples to something like:

#' # Retrieve a list of surveys
#' surveys <- all_surveys()
#'
#' # Retrieve a single survey using your `surveys` dataframe:
#' my_survey <- fetch_survey(surveyID = surveys$id[6])
#'
#' # Retrieve a single survey if you know the survey ID as a string:
#' my_survey <- fetch_survey(surveyID = "SV_xxxxxxxxxxxxxxx")
#'
#' # Retrieve your survey specifying other args:
#' my_survey <- fetch_survey(
#'   surveyID = surveys$id[6],
#'   start_date = "2018-01-01",
#'   end_date = "2018-01-31",
#'   limit = 100,
#'   label = TRUE,
#'   unanswer_recode = 999,
#'   verbose = TRUE,
#'   # Manually override EndDate to be a character vector
#'   col_types = readr::cols(EndDate = readr::col_character())
#' )

Are you interested in updating this PR, or doing a separate one for this change?