printf.9: Update the use example of the %D conversion specifier

The output of hexadecimal bytes are in lowercase. Update the example to
reflect the reality.

Reviewed by: gbe (manpages)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39543

(cherry picked from commit 722b7589cca704670c3c532bdd0f5d46675df085)