fix: docs page - installing slate

[BarrBozzO]

Description Documentation page(Installing slate) fix: proper use of useState for an editor initialization.

A clear and concise description of what this pull request solves. (Please do not just link to a long issue thread. Instead include a clear description here or your pull request will likely not be reviewed as quickly.)

Issue Incorrect\Before:

const editor = useState(() => withReact(createEditor())) 

Correct\After:

const [editor] = useState(() => withReact(createEditor())) 

Example

Context

Checks

• [ ] The new code matches the existing patterns and styles.
• [ ] The tests pass with yarn test.
• [ ] The linter passes with yarn lint. (Fix errors with yarn fix.)
• [ ] The relevant examples still work. (Run examples with yarn start.)
• [ ] You've added a changeset if changing functionality. (Add one with yarn changeset add.)

[changeset-bot[bot]]

⚠️ No Changeset found

Latest commit: 13f6a7ec77b97b6caa6669fe060e275194ddf5b4

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[dylans]

Closing in favor of #5097 which has the same fix but actually filled in the issue template.