chore(deps): remove dev dependency @types/cron

[sheerlox]

As of version 2.4.0 of cron, types are bundled with the library.

This PR removes the @types/cron dev dependency in package.json, and removes the installation instruction about that same package in README.md, streamlining the installation of @nestjs/schedule to a single line.

Note: this PR targets the Renovate Bot PR #1355 to make it easier for maintainers to merge these related changes.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe: Development dependencies & documentation update

Does this PR introduce a breaking change?

• Yes
• No

Other information

Related PR: DefinitelyTyped/DefinitelyTyped#66164

Commands ran (successfully) to test the changes:

$ npm run build
$ npm run test:integration

[kamilmysliwiec]

lgtm