docs: fix elicitation example to use requestedSchema - #2978

What

Fixes the elicitation request example in docs/docs/learn/client-concepts.mdx.
The example used a schema field:

{
  method: "elicitation/create",
  params: {
    message: "...",
    schema: { ... }   // ❌
  }
}

but the canonical field name is requestedSchema. This matches:

• schema/draft/schema.ts / schema/2025-06-18/schema.ts (ElicitRequest.params.requestedSchema)
• schema/draft/examples/ElicitRequest/elicitation-request.json
• the specification page docs/specification/2025-06-18/client/elicitation.mdx

Why

A reader copying the learn-page example would send params.schema, which a
spec-compliant client ignores (it reads params.requestedSchema), so the
elicitation form would be empty. The fix aligns the learn doc with the spec.

Change

One-line field rename schema: → requestedSchema: in the elicitation
components example. No other content changed.