Give better hint for question_checkbox() in try_again message

[gadenbuie]

Sets the default value of try_again in question_checkbox() to the message suggested in #731. This gives a better hint that there could be more than one answer and only when retries are allowed.

question_checkbox(
  "What's the best package?",
  answer("tidyverse", correct = TRUE),
  answer("base", correct = TRUE),
  allow_retry = TRUE
)
#> Question: "What's the best package?"
#>   type: "learnr_checkbox"
#>   allow_retry: TRUE
#>   random_answer_order: FALSE
#>   answers:
#>     ✔: "tidyverse"
#>     ✔: "base"
#>   messages:
#>     correct: "Correct!"
#>     incorrect: "Incorrect"
#>     try_again: "Incorrect. Be sure to select every correct answer." 

Screen.Recording.2022-09-08.at.3.32.53.PM.mov