DRI3: fix off-by-one DRI3 opcodes in description

The actual protocol and implementation details are unaffected - both here and in xcb. Only the description/diagram section is off.

Noticed while skimming through the new 1.3 additions.

Seemingly first introduced with @lfrb modifiers work and the follow-up 1.3 work by @ashafer

/cc @ofourdan feel free to shepherd this in