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