SQL/JSON: Fix a paragraph in JSON_TABLE documentation

SQL/JSON: Fix a paragraph in JSON_TABLE documentation

Using <replaceable>text</replaceable> inside parantheses is not a
common or good style, so rephrase a sentence to avoid that style.
Also rephrase the text in that paragraph a bit while at it.

Reported-by: Marcos Pegoraro <marcos@f10.com.br>
Author: Jian He <jian.universality@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/CAB-JLwZqH3Yec6Kz-4-+pa0ZG9QJBsxjJZwYcMZYzEDR_fXnKw@mail.gmail.com