[bugfix] fix slice init length

[cuishuang]

Description

The intention here should be to initialize a slice with a capacity of diff rather than initializing the length of this slice.

The online demo: https://go.dev/play/p/q1BcVCmvidW

Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: [ ] -> [x]

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[NyaaaWhatsUpDoc]

thanks for the contribution! before merge can you go through confirm some of our checklist boxes please? :)

[cuishuang]

thanks for the contribution! before merge can you go through confirm some of our checklist boxes please? :)

Thanks. Modified.

[cuishuang]

I think the change is minor and the issues fixed are very clear according to the online demo: https://go.dev/play/p/q1BcVCmvidW