qemu-doc: Fix ivshmem usage example with shm=...

The example suggests you can omit "shm".  This isn't true; you must
specify exactly one of "shm", "chardev", "memdev".  Fix it.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <1448384789-14830-3-git-send-email-armbru@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>

qemu-doc.texi

@@ -1256,7 +1256,7 @@ zero-copy communication to the application level of the guests.  The basic
 syntax is:
 
 @example
-qemu-system-i386 -device ivshmem,size=@var{size}[,shm=@var{shm-name}]
+qemu-system-i386 -device ivshmem,size=@var{size},shm=@var{shm-name}
 @end example
 
 If desired, interrupts can be sent between guest VMs accessing the same shared