fix: Update the links to enabling-decorators

[pranavrajs]

The link to decorators in the installation documentation is incorrect. It generates the link as https://mobx.js.org/enabling-decorators.md instead of https://mobx.js.org/enabling-decorators.html. This PR fixes it.

Note: I've tested this locally, seems to be working fine.

[changeset-bot]

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Latest commit: 6887514

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[kubk]

@pranavrajs Thank you