[odin/en] Fix various mistakes and clarify verbiage

[CollinEMac]

• I solemnly swear that this is all original content of which I am the original author
• Content changes are aimed at intermediate to experienced programmers (this is a poor format for explaining fundamental programming concepts)
• 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)
• If you've changed any part of the YAML Frontmatter, make sure it is formatted according to CONTRIBUTING.md

Related issue: #5370

[gingerBill]

By the way, I give you permission to take things from Odin's official demo.odin file if you want. That was originally Odin's equivalent of "learnxinyminutes" for the language, before we had the Overview and even a book written for Odin.