qemu-options: trivial fix for -mon args help

It is the (implied sub)option name which is optional, not
the value of that (sub)option, make it so in the help output.
(Introduced by commit 22a0e04b)

Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

qemu-options.hx

@@ -2539,9 +2539,9 @@ Like -monitor but opens in 'control' mode.
 ETEXI
 
 DEF("mon", HAS_ARG, QEMU_OPTION_mon, \
-    "-mon chardev=[name][,mode=readline|control][,default]\n", QEMU_ARCH_ALL)
+    "-mon [chardev=]name[,mode=readline|control][,default]\n", QEMU_ARCH_ALL)
 STEXI
-@item -mon chardev=[name][,mode=readline|control][,default]
+@item -mon [chardev=]name[,mode=readline|control][,default]
 @findex -mon
 Setup monitor on chardev @var{name}.
 ETEXI