Fix the list of hexadecimal characters in isHexadecimalDigit.adoc

[edgar-bonet]

The documentation of isHexadecimalDigit() states:

Analyse if a char is an hexadecimal digit (A-F, 0-9).

Contrary to what is implied here, the lowercase letters “a” through “f” are also identified as hexadecimal digits by this function.

This pull request completes the list of characters, and sorts them by ASCII order: 0–9, A–F, a–f.