docs: add new HTTP Error Codes 50068 (v13)

[aiko-chan-ai]

Please describe the changes this PR makes and why it should be merged:
After I started debugging with the error 50067 Invalid request origin (not in the documentation), I saw a new error 50068

Here is a PR that adds this error code
Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating
• This PR only includes non-code changes, like changes to documentation, README, etc.

[aiko-chan-ai]

#8272 I made a mistake when doing pr with main branch. Sorry for this problem 😢

[Jiralite]

#8272 I made a mistake when doing pr with main branch. Sorry for this problem 😢

You can change the branch a pull request targets (if you look at the linked pull request before it was closed, you can see a message stating this occurred).