docs(mongo): pass ref in PropOptions root

[percimar]

In the mongoose @Prop decorator code snippets, the ref option is being passed nested inside the type option. However, as seen in the implementation here, ref is expected at the root of PropOptions, not nested within PropOptions.type.

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• The commit message follows our guidelines: https://github.com/nestjs/docs.nestjs.com/blob/master/CONTRIBUTING.md

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
• Docs
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

ref is being passed nested inside type, causing population to not work.

What is the new behavior?

ref is being passed correctly, causing population to work correctly.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[kamilmysliwiec]

Could you please provide a minimum reproduction repository (Git repository/StackBlitz/CodeSandbox project)?