Update Joi example to current version (16.*) by fixing deprecated Joi…

[murbanowicz]

….validate usage

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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?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] Other... Please describe:

What is the current behavior?

Docs show example which is using deprecated Joi.validate() as per hapijs/joi#1941

What is the new behavior?

Docs show example with current way of usage which is calling validate() directly on schema.

[kamilmysliwiec]

Thank you!

[murbanowicz]

No problem! Thank you for great work on Nest :)

[kamilmysliwiec]

❤️