docs: fix response body for local provider

[Syafiqjos]

Fix response body from tokens to token in quick-start docs in local and refresh providers.

As written in the nuxt-auth example codes, the correct return body is token instead of tokens as tokens gives Error: Invalid reference token error.

File References:

• https://github.com/sidebase/nuxt-auth-example/blob/add-local-provider/server/api/auth/login.post.ts
• https://github.com/sidebase/nuxt-auth/blob/main/playground-local/server/api/auth/login.post.ts

Checklist:

• issue number linked above after pound (#)
  • replace "Closes " with "Contributes to" or other if this PR does not close the issue
• manually checked my feature / checking not applicable
• wrote tests / testing not applicable
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