package/system/mtd: fix usage message

Minor fix in the usage message on the explanation of the -p option.

Signed-off-by: Paul Wassi <p.wassi@gmx.at>

package/system/mtd/src/mtd.c

@@ -732,7 +732,7 @@ static void usage(void)
 	"        -d <name>               directory for jffs2write, defaults to \"tmp\"\n"
 	"        -j <name>               integrate <file> into jffs2 data when writing an image\n"
 	"        -s <number>             skip the first n bytes when appending data to the jffs2 partiton, defaults to \"0\"\n"
-	"        -p                      write beginning at partition offset\n"
+	"        -p <number>             write beginning at partition offset\n"
 	"        -l <length>             the length of data that we want to dump\n");
 	if (mtd_fixtrx) {
 	    fprintf(stderr,