Variables and Operators lesson: Fix typo in introduction paragraph

[hamacodes]

First sentence of introduction paragraph had a typo "learnt" instead of "learned"

Because

Introduction paragraph has a type in the first sentence. Fixing it will allow a better flow for learner's who might lose focus due to typo.

This PR

Changed word typo "learnt" to "learned"

Issue

N/A

Closes #XXXXX

Additional Information

None

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[MaoShizhong]

While the Brit in me hates to see this changed, our style guide does enforce American English spelling so it does need to change 😭