format-table: use right field for display

Since .timespan and .timestamp are unionized on top of each other this
doesn't actually matter, but it is still more correct to address it
under it's correct name.

src/shared/format-table.c

@@ -884,7 +884,7 @@ static const char *table_data_format(TableData *d) {
                 if (!p)
                         return NULL;
 
-                if (!format_timespan(p, FORMAT_TIMESPAN_MAX, d->timestamp, 0))
+                if (!format_timespan(p, FORMAT_TIMESPAN_MAX, d->timespan, 0))
                         return "n/a";
 
                 d->formatted = TAKE_PTR(p);