fix docs css.loaderOptions.css.localsConvention

[negibouze]

Moved 'localsConvention' to the same level as 'modules'.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Underlying tools
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

Other information:
I only tested on macOS.

logs from css-loader's

[Function: validate] {
  schema: {
    additionalProperties: false,
    properties: {
      url: [Object],
      import: [Object],
      modules: [Object],
      sourceMap: [Object],
      importLoaders: [Object],
      localsConvention: [Object],
      onlyLocals: [Object]
    },
    type: 'object'
  },
  errors: null,
  refs: {},
  refVal: [ [Circular] ],
  root: [Circular]
}

[vue-bot]

Hey @negibouze, thank you for your time and effort spent on this PR, contributions like yours help make Vue better for everyone. Cheers! 💚