docs: fix typo in create-sandbox (it's -> its)

[Gao1ng]

Purpose (TL;DR) - mandatory

Fix a grammatical typo in docs/concepts/sandboxes/create-sandbox.md: "it's own" should be "its own" (possessive pronoun, not the contraction "it is").

How to verify - mandatory

1. View docs/concepts/sandboxes/create-sandbox.md
2. Confirm the text reads: "Creates a new sandbox object with its own set of spies, stubs, and mocks."

Checklist for author

• npm run lint passes
• References to standard library functions are cached.