Updated cookies.md with register signature

[fedotarte]

at 9.0.4 we should return a callback, or we've got a ts error:

TS2345: Argument of type 'typeof fastifyCookie' is not assignable to parameter of type 'FastifyPluginCallback | FastifyPluginAsync | Promise{ default: FastifyPluginCallback ; }> | Promise...>'.

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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?

for @nestjs/platform-fastify 10.0.2 and fastify/cookies 9.0.4 we should return a callback inside register, or we've got a ts error:

TS2345: Argument of type 'typeof fastifyCookie' is not assignable to parameter of type 'FastifyPluginCallback | FastifyPluginAsync | Promise{ default: FastifyPluginCallback ; }> | Promise...>'.

Issue Number: N/A

What is the new behavior?

it's about register cookies plugin for nest fastify plattform

Does this PR introduce a breaking change?

• Yes
• No

Other information