Update block-content.md

[arjenblokzijl]

Hi,

A co-worker of mine was looking for some examples regarding named blocks. Since it only had one (minor - default) example I've gone ahead and added another one. Feedback appreciated!

🙏

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[elwayman02]

@jenweber @locks is anything blocking this from being merged?

[IgnaceMaes]

Closing in favour of https://github.com/ember-learn/guides-source/pull/1958.

Thanks for the initial spike on these docs! If you feel like anything is missing, please open a new PR.