Update parser README to include parse_jsontable.c

The README was missing parse_jsontable.c which handles JSON_TABLE.
Oversight in de3600452b.

Author: Karthik S <karthikselvaam@gmail.com>
Discussion: https://postgr.es/m/CAK4gQD9gdcj+vq_FZGp=Rv-W+41v8_C7cmCUmDeu=cfrOdfXEw@mail.gmail.com
Backpatch-through: 17

src/backend/parser/README

@@ -20,6 +20,7 @@ parse_cte.c	handle Common Table Expressions (WITH clauses)
 parse_expr.c	handle expressions like col, col + 3, x = 3 or x = 4
 parse_enr.c	handle ephemeral named rels (trigger transition tables, ...)
 parse_func.c	handle functions, table.column and column identifiers
+parse_jsontable.c handle JSON_TABLE
 parse_merge.c	handle MERGE
 parse_node.c	create nodes for various structures
 parse_oper.c	handle operators in expressions