Fix usage to show that vendor: is optional in the -m parameter

Corresponding to flashrom svn r450.
Signed-off-by: Peter Stuge <peter@stuge.se>
Acked-by: Peter Stuge <peter@stuge.se>

flashrom.8

@@ -3,7 +3,7 @@
 flashrom \- read, write, and erase BIOS/ROM/flash chips
 .SH SYNOPSIS
 .B flashrom \fR[\fB\-rwvEVfLhR\fR] [\fB\-c\fR chipname] [\fB\-s\fR exclude_start] [\fB\-e\fR exclude_end]
-         [\fB-m\fR vendor:part] [\fB-l\fR file.layout] [\fB-i\fR image_name] [file]
+         [\fB-m\fR [vendor:]part] [\fB-l\fR file.layout] [\fB-i\fR image_name] [file]
 .SH DESCRIPTION
 .B flashrom
 is a utility for reading, writing, and erasing flash ROM chips.