Improve documentation example for jsonpath like_regex operator

Improve documentation example for jsonpath like_regex operator

Make sample like_regex match string values of the root object instead of the
whole document.  The corrected example seems to represent a more relevant
use case.

Backpatch to 12, when jsonpath was introduced.

Discussion: https://postgr.es/m/13440f8b-4c1f-5875-c8e3-f3f65606af2f%40xs4all.nl
Author: Erik Rijkers
Reviewed-by: Michael Paquier, Alexander Korotkov
Backpatch-through: 12