fix(docs): replace npm with pnpm

[IlyaSemenov]

The README misleads that one must use npm. In fact, the project uses pnpm, and it won't even install with npm:

npm ERR! code ERESOLVE
npm ERR! ERESOLVE could not resolve
npm ERR! 
npm ERR! While resolving: @nuxt/test-utils@3.7.4
npm ERR! Found: vitest@0.34.6
npm ERR! node_modules/vitest
npm ERR!   dev vitest@"^0.34.1" from the root project
npm ERR! 
npm ERR! Could not resolve dependency:
npm ERR! peerOptional vitest@"^0.30.0 || ^0.31.0 || ^0.32.0 || ^0.33.0" from @nuxt/test-utils@3.7.4
npm ERR! node_modules/@nuxt/test-utils
npm ERR!   dev @nuxt/test-utils@"^3.7.0" from the root project
npm ERR! 
npm ERR! Conflicting peer dependency: vitest@0.33.0
npm ERR! node_modules/vitest
npm ERR!   peerOptional vitest@"^0.30.0 || ^0.31.0 || ^0.32.0 || ^0.33.0" from @nuxt/test-utils@3.7.4
npm ERR!   node_modules/@nuxt/test-utils
npm ERR!     dev @nuxt/test-utils@"^3.7.0" from the root project
npm ERR! 
npm ERR! Fix the upstream dependency conflict, or retry
npm ERR! this command with --force or --legacy-peer-deps
npm ERR! to accept an incorrect (and potentially broken) dependency resolution.

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