Fix typo: lenghth --> length.

[heijp06]

Fix a small typo in introduction.md for the lasagna-master exercise.

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[kotp]

There is another occurrence of this typo as well, if you would like to add that change to this PR.

• concepts/functions/introduction.md
• exercises/concept/lasagna-master/.docs/introduction.md

[heijp06]

@kotp, fixed that as well. Thanks for pointing it out.

[vaeng]

Thanks for fixing that typo.

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