doc: Update pg_size_pretty documentation about petabytes support

Missing documentation update for ca2e447.

Discussion: https://www.postgresql.org/message-id/CAApHDvrCwMgSD_93LZr4CLMas8Hc61fXAQ-Cd4%3D%2ByoRfHnYbJA%40mail.gmail.com

doc/src/sgml/func.sgml

@@ -26901,7 +26901,7 @@ postgres=# SELECT * FROM pg_walfile_name_offset((pg_backup_stop()).lsn);
        </para>
        <para>
         Converts a size in bytes into a more easily human-readable format with
-        size units (bytes, kB, MB, GB or TB as appropriate).  Note that the
+        size units (bytes, kB, MB, GB, TB, or PB as appropriate).  Note that the
         units are powers of 2 rather than powers of 10, so 1kB is 1024 bytes,
         1MB is 1024<superscript>2</superscript> = 1048576 bytes, and so on.
        </para></entry>