remove $ prompt symbols from installation commands in README.md

[gap-editor]

Fixes #5638

This PR removes the $ prompt symbols from the installation commands in the README to make them directly copy-pasteable. When users try to copy-paste commands that start with $, they need to manually remove this character, which creates an unnecessary inconvenience.

Changes:

• Removed $ from npm install @openzeppelin/contracts command
• Removed $ from forge install OpenZeppelin/openzeppelin-contracts command

PR Checklist

• Tests (N/A - documentation change only)
• Documentation
• Changeset entry (run npx changeset add)

[changeset-bot]

⚠️ No Changeset found

Latest commit: 3bf9bfc

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