Remove word "singleton" out of jsonpath docs

Word "singleton" is hard for user understanding, especially taking into account
there is only one place it's used in the docs and there is even no definition.
Use more evident wording instead.

Discussion: https://postgr.es/m/23737.1556550645%40sss.pgh.pa.us

Author: akorotkov

doc/src/sgml/func.sgml

@@ -11721,7 +11721,7 @@ table2-mapping
     converting its elements into an SQL/JSON sequence before performing
     this operation. Besides, comparison operators automatically unwrap their
     operands in the lax mode, so you can compare SQL/JSON arrays
-    out-of-the-box. Arrays of size 1 are interchangeable with a singleton.
+    out-of-the-box. An array of size 1 is considered equal to its sole element.
     Automatic unwrapping is not performed only when:
     <itemizedlist>
      <listitem>