remove Teach Yourself Web3 link in README

[Melvillian]

This community is no longer active (even though the site is up). It should be removed so that readers are not lead astray.

Checklist

• [X] The URL is not already present in the list (check with CTRL/CMD+F in the raw markdown file).
• [X] Each description starts with an uppercase character and ends with a period.
  Example: solc-js - JavaScript bindings for the compiler.
• [X] Drop all A / An prefixes at the start of the description.
• [X] Avoid using the word Solidity in the description.

[bkrem]

Thank you 🙏