Fix Typo: Remove duplicated "not"

[takahirox]

[javagl]

@lexaknyazev When things affect the core spec, then I'd hesitate to just review/merge anything without your approval (a similar case as in https://github.com/KhronosGroup/glTF/pull/2351 ). If you say that things like these (uncontroversial typos) are OK to be handled by anyone, then these PRs could be worked off more quickly.

[javagl]

@lexaknyazev If you agree that things like

• this
• https://github.com/KhronosGroup/glTF/pull/2351
• https://github.com/KhronosGroup/glTF/pull/2246

can be merged at any point in time and by others than you, then this could help work off the PR queue.

But maybe there is a plan/schedule that should be followed.

For example: If the plan is that such PRs are consistently tagged PRs specification and added to https://github.com/KhronosGroup/glTF/milestone/4 , so that they can be merged "in bulk" (and maybe to create a change log from that), then it would be good to know this process.

(If there is such a process, then I'd add a comment to such PRs, summarizing that: "This is approved and will be merged a part of [...]", just to let those know who opened them)