chore: fix typo in docs for typescript chapter

[soomtong]

in reading docs I found a typo issue to change.
it seems 'server' not 'fastify'.

const server = fastify()
// stuffs...
server().register(plugin, {}) // Error - options object is missing required properties
server().register(plugin, { option1: '', option2: true }) // OK - options object contains required properties

Checklist

• documentation is changed or added

[Uzlopak]

LGTM

[mcollina]

lgtm

[github-actions]

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