[docs-infra] Fix Vale config for TypeScript references

[oliviertassinari]

Vale failed to report this case. It's quick to fix, so I went for it. Saw a regression in https://github.com/mui/material-ui/pull/43218#discussion_r1759614060. Now, Vale correctly reports the error back.

[mui-bot]

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• docs/data/material/migration/upgrade-to-v6/migrating-to-pigment-css.md

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[oliviertassinari]

Finger crossed 🤞, it won't break MUI X 😄