doc: fixes to documentation for `gleam/dynamic/decode`

[isaacharrisholt]

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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.

[isaacharrisholt]

docs: minor corrections to dynamic/decode

Yep! All done