doc/mkimage.1: Fix -D/-f options formatting

Signed-off-by: Horst Kronstorfer <hkronsto@frequentis.com>

doc/mkimage.1

@@ -82,12 +82,12 @@ Set XIP (execute in place) flag.
 .B Create FIT image:
 
 .TP
-.BI "\-D "dtc option"
+.BI "\-D [" "dtc options" "]"
 Provide special options to the device tree compiler that is used to
 create the image.
 
 .TP
-.BI "\-f "fit-image.its"
+.BI "\-f [" "image tree source file" "]"
 Image tree source file that describes the structure and contents of the
 FIT image.