📚 Fix typo

[blakeNaccarato]

Just a minor typo I saw while reading MathJax docs. Changed "ensurea" to "ensures".

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Codecov Report

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

Project coverage is 90.07%. Comparing base (95e5ca7) to head (845a95e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #911 +/- ## ======================================= Coverage 90.07% 90.07% ======================================= Files 24 24 Lines 3405 3405 ======================================= Hits 3067 3067 Misses 338 338 ``` | [Flag](https://app.codecov.io/gh/executablebooks/MyST-Parser/pull/911/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks) | Coverage Δ | | |---|---|---| | [pytests](https://app.codecov.io/gh/executablebooks/MyST-Parser/pull/911/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks) | `90.07% <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=executablebooks#carryforward-flags-in-the-pull-request-comment) to find out more.

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