docs: Update README.md w new references to @cypress/grep

[builtbylane]

User facing changelog

Update @cypress/grep README.md with correct references to the moved package @cypress/grep

PR Tasks

• Have tests been added/updated?
• Has the original issue (or this PR, if no issue exists) been tagged with a release in ZenHub? (user-facing changes only)
• Has a PR for user-facing changes been opened in cypress-documentation?
• Have API changes been updated in the type definitions?

[cypress-bot]

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[CLAassistant]

All committers have signed the CLA.

[jordanpowell88]

Thank you!

[jennifer-shehane]

@builtbylane Thanks for the contribution! Could you please sign our CLA?

[builtbylane]

@jennifer-shehane signed