Explicitly non-digit characters in testing exercise (#2723)

Students sometimes ask about this so I figure it'd be good to specify in
the text.

Author: randomPoison

src/testing/exercise.md

@@ -8,7 +8,8 @@ The [Luhn algorithm](https://en.wikipedia.org/wiki/Luhn_algorithm) is used to
 validate credit card numbers. The algorithm takes a string as input and does the
 following to validate the credit card number:
 
-- Ignore all spaces. Reject numbers with fewer than two digits.
+- Ignore all spaces. Reject numbers with fewer than two digits. Reject letters
+  and other non-digit characters.
 
 - Moving from **right to left**, double every second digit: for the number
   `1234`, we double `3` and `1`. For the number `98765`, we double `6` and `8`.