docs: fix typos in ONBOARDING.md

[Prince-ES]

Description

Typo in ONBOARDING.md
(i) prerequisits -> prerequisites
(i) instace ->instance

Fixes #762

Type of change

Spelling fix.

• Refactor (does not change functionality, e.g. code style improvements, linting)
• Documentation update

How Has This Been Tested?

not applicable.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

Maintainer Checklist

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tag the PR with the appropriate labels

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