fix(core): update PortableTextInput doc reference

[pedrobonamin]

Description

Internal TS reference for PortableTextInput customisation returns a 404.

https://www.sanity.io/docs/portable-text-features

This PR updates to the correct place in the documentation.

https://www.sanity.io/docs/customizing-the-portable-text-editor

What to review

Testing

Notes for release

update PortableTextInput doc reference

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[github-actions]

No changes to documentation

[github-actions]

Component Testing Report Updated Feb 7, 2024 12:12 PM (UTC)

File	Status	Duration	Passed	Skipped	Failed
comments/CommentInput.spec.tsx	✅ Passed (Inspect)	33s	15	0	0
formBuilder/ArrayInput.spec.tsx	✅ Passed (Inspect)	6s	3	0	0
formBuilder/inputs/PortableText/Annotations.spec.tsx	✅ Passed (Inspect)	12s	3	0	0
formBuilder/inputs/PortableText/copyPaste/CopyPaste.spec.tsx	✅ Passed (Inspect)	13s	4	2	0
formBuilder/inputs/PortableText/Decorators.spec.tsx	✅ Passed (Inspect)	13s	6	0	0
formBuilder/inputs/PortableText/FocusTracking.spec.tsx	✅ Passed (Inspect)	33s	15	0	0
formBuilder/inputs/PortableText/Input.spec.tsx	✅ Passed (Inspect)	18s	9	0	0
formBuilder/inputs/PortableText/ObjectBlock.spec.tsx	❌ Failed (Inspect)	1m 7s	17	0	1
formBuilder/inputs/PortableText/Styles.spec.tsx	✅ Passed (Inspect)	13s	6	0	0
formBuilder/inputs/PortableText/Toolbar.spec.tsx	❌ Failed (Inspect)	25s	8	0	1