doc: Fix typo in section "WAL configuration"

pg_stat_io has an attribute named fsync_time, not sync_time.

Oversight in 2f70871.

Discussion: https://postgr.es/m/Z7RkQ0EfYaqqjgz/@ip-10-97-1-34.eu-west-3.compute.internal

Author: michaelpq

doc/src/sgml/wal.sgml

@@ -813,7 +813,7 @@
    When <xref linkend="guc-track-io-timing"/> is enabled, the total
    amounts of time <function>XLogWrite</function> writes and
    <function>issue_xlog_fsync</function> syncs WAL data to disk are counted as
-   <varname>write_time</varname> and <varname>sync_time</varname> in
+   <varname>write_time</varname> and <varname>fsync_time</varname> in
    <xref linkend="pg-stat-io-view"/> for the <varname>object</varname>
    <literal>wal</literal>, respectively.
    <function>XLogWrite</function> is normally called by