[golang/en] improve explanations of structs, maps and sets

[wagslane]

• I solemnly swear that this is all original content of which I am the original author
• Pull request title is prepended with [language/lang-code] (example [python/fr] for Python in French or [java] for multiple Java translations)
• Pull request touches only one file (or a set of logically related files with similar changes made)
• Content changes are aimed at intermediate to experienced programmers (this is a poor format for explaining fundamental programming concepts)
• If you've changed any part of the YAML Frontmatter, make sure it is formatted according to CONTRIBUTING.md

[ANTALLIS]

I see that you've inserted a little boot.dev promotion in there.

[wagslane]

I see that you've inserted a little boot.dev promotion in there.

Ya ain't wrong xD Hustlin' out here.

But in all seriousness I thought it would be fine as there were two other resources listed, and the content in the linked Boot.dev course is all free