docs: Fix documentation of `ValidationError::ConstTypeError`

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.99%. Comparing base (c05edd3) to head (7085da4).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1227 +/- ## ======================================= Coverage 86.99% 86.99% ======================================= Files 100 100 Lines 18975 18975 Branches 16992 16992 ======================================= Hits 16508 16508 Misses 1689 1689 Partials 778 778 ``` | [Flag](https://app.codecov.io/gh/CQCL/hugr/pull/1227/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CQCL) | Coverage Δ | | |---|---|---| | [rust](https://app.codecov.io/gh/CQCL/hugr/pull/1227/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CQCL) | `86.48% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CQCL#carryforward-flags-in-the-pull-request-comment) to find out more.

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