docs-bug(Stepper): fix typo at accessible notes

[raisiqueira]

Documentation Feedback

An error found at Accessibility notes at Mat Stepper documentation.

Prefer verticl steppers when building for small screen sizes,[...]

instead of:

Prefer vertical steppers when building for small screen sizes,[...]

Affected documentation page

https://v12.material.angular.io/components/stepper/overview#accessibility

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