Doc: fix missing comma at the end of a line.

Backpatch to 17, where the line was added.

Reported by Noboru Saito while he was working on translating the file
into Japanese.

Discussion: https://postgr.es/m/20250417.203047.1321297410457834775.ishii%40postgresql.org
Reported-by: Noboru Saito <noborusai@gmail.com>
Reviewed-by: Daniel Gustafs <daniel@yesql.se>
Backpatch-through: 17

Author: tatsuo-ishii

doc/src/sgml/libpq.sgml

@@ -5334,7 +5334,7 @@ unsigned char *PQunescapeBytea(const unsigned char *from, size_t *to_length);
    <xref linkend="libpq-PQprepare"/>,
    <xref linkend="libpq-PQexecPrepared"/>,
    <xref linkend="libpq-PQdescribePrepared"/>,
-   <xref linkend="libpq-PQdescribePortal"/>
+   <xref linkend="libpq-PQdescribePortal"/>,
    <xref linkend="libpq-PQclosePrepared"/>, and
    <xref linkend="libpq-PQclosePortal"/>
    respectively.