Clarify: Elements and Tags

[Fiattarone]

I didn't see any examples of self-closing tags, or the mention of the term "self-closing tags," which from my experience is a more prevalent term than "empty elements".

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1. Because:

Closes #XXXXX

• Because learners need exposure to a potentially more prevalent term for the topic at hand with a visual example.

2. This PR:

• Added clarification for empty elements/self-closing tags.

3. Additional Information:

[Fiattarone]

Just wanted to add, the first mention of self-closing tags other than what I've contributed here is in CSS Foundations--I think it will be more beneficial for learners to see this earlier on rather than be exposed without an explanation.