doc/userguide: fix within-distance pointer graphics in payload-keywords doc

Redmine ticket: #8261

According to [1], the within pointer (if combined with distance)
includes the distance pointer, which is not clearly visible in the
graphic.

Fixed this in a new graphic by some GIMP arts.

PS: Special thanks to one of our team members Annika C. for initially
spotting this!

[1] https://forum.suricata.io/t/is-within-affected-by-distance/1688

Author: e-cite