Bump is-svg from 4.2.1 to 4.3.1 in /docs/src (#8177)

Bumps [is-svg](https://github.com/sindresorhus/is-svg) from 4.2.1 to 4.3.1.
- [Release notes](https://github.com/sindresorhus/is-svg/releases)
- [Commits](sindresorhus/is-svg@v4.2.1...v4.3.1)

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