doc: fixes to documentation for gleam/dynamic/decode

[isaacharrisholt]

Couple of fixes to the documentation for the decode module:

• Replaced assert with let assert as the former is not currently valid Gleam code.
  • I did this work in a separate commit so it can be easily reverted when this syntax becomes valid.
• Fixed a syntax error.
• Removed a reference to decode.then from an example that doesn't use it.

[lpil]

Could you undo the assert change please? We will likely have assert by the time this is release 🙏