Bump mixin-deep from 1.3.1 to 1.3.2 in /demo

Bumps [mixin-deep](https://github.com/jonschlinkert/mixin-deep) from 1.3.1 to 1.3.2.
- [Release notes](https://github.com/jonschlinkert/mixin-deep/releases)
- [Commits](jonschlinkert/mixin-deep@1.3.1...1.3.2)

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Author: dependabot[bot]