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

Author: akorotkov

doc/src/sgml/func.sgml

@@ -13238,9 +13238,10 @@ strict $.**.HR
      is a JSON path string literal, written according to the rules given in
      <xref linkend="datatype-jsonpath"/>.  This means in particular that any
      backslashes you want to use in the regular expression must be doubled.
-     For example, to match strings that contain only digits:
+     For example, to match string values of the root document that contain
+     only digits:
 <programlisting>
-'$ ? (@ like_regex "^\\d+$")'
+$.* ? (@ like_regex "^\\d+$")
 </programlisting>
     </para>