[unpleasent] spell check

[longye-tian]

Dear John @jstac and Matt @mmcky ,

This pull request reviews the spellings of the lecture unpleasant.md and check whether we need to add example admonition according to #532 and #520 .

In this pull request, we

• change 'portolio' to 'portolio'
• change 'it's supply' to 'its supply'
• change '... that let's us' to '... that lets us'
• we find no examples to include in the example admonition

Best ❤️ Longye

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