Fix typos, penguin links, and phrasing

[mike-ivs]

Part of https://github.com/carpentries-incubator/deep-learning-intro/issues/482

Supplementary material

• There are just a few small typos, wording tweaks, and a url fix (pesky non-static internet!)

Episode 2 - Classification by a neural network using Keras

• updates to Palmer penguin links
• tweaks to various sentences

[github-actions[bot]]

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