specs: restore github-pages deploy by removing deprecated mdBook config key

[Copilot]

Description

The github-pages / deploy (pull_request) job failed during mdbook build specs because specs/book.toml used a removed mdBook config field (multilingual). This PR removes the incompatible field so the docs build can proceed on the current mdbook version used in CI.

Changes

• Root cause in CI

  • mdbook v0.5.x rejects [book].multilingual with: unknown field multilingual.

• Minimal fix

  • Removed multilingual = false from specs/book.toml.

• Scope

  • Single-file docs config change; no workflow logic or application code changes.

Patch excerpt

[book]
authors = ["Celestia Labs"]
language = "en"
src = "src"
title = "Celestia Node Specification"