Fix ItemSelectionInput instruction to be clearer to students.

[seanlip]

Describe the bug
The text for ItemSelectionInput is not clear to students.

To Reproduce
Steps to reproduce the behavior:

1. Go to an item selection interaction (see e.g. Multiplication 1). See screenshot below.

Observed behavior
The wording "Please select at least one choice." is confusing to students. They keep on choosing just one right answer and leaving the other right answers on the table, which results in them getting feedback that their answer is not correct. This leads to frustration.

Expected behavior
Change the prompt to "Please select all correct choices”.

Demonstration

[KolliAnitha]

Verified the issue in 0.8-alpha-091b45a188. 'Please select all correct choices' is displayed for multi selection inputs.