Add --output-format option to manpage.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/fio.1 b/fio.1
index 33b26cc..b746da1 100644
--- a/fio.1
+++ b/fio.1
@@ -20,6 +20,9 @@
 .BI \-\-output \fR=\fPfilename
 Write output to \fIfilename\fR.
 .TP
+.BI \-\-output-format \fR=\fPformat
+Set the reporting format to \fInormal\fR, \fIterse\fR, or \fIjosn\fR.
+.TP
 .BI \-\-runtime \fR=\fPruntime
 Limit run time to \fIruntime\fR seconds.
 .TP