docs: Update @cypress/grep README install instructions

[will-banked]

• Closes: N/A

User facing changelog

Swaps out references of 'cypress-grep' for '@cypress/grep' in the installation section of the @cypress/grep README

Additional details

N/A

Steps to test

I've just migrated from cypress-grep to @cypress/grep and wanted to update the README with things I needed to update to get the new package working again.

How has the user experience changed?

N/A

PR Tasks

I don't think there are any particular tasks that will need to happen for a README change.

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

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[jordanpowell88]

Thank you!

[will-banked]

No problem :)