docs: fix broken CHANGELOG links reported by check-links (#4467)

* docs: fix broken links in CHANGELOG

Fixes #4465.

Addresses the five 404s reported by the check-links workflow:

- pull/1670 entry: remove stray bracket and leading paren so the
  markdown link parses correctly.
- aws/ecs.md: point to the current location in the semantic-conventions
  repo. Semantic conventions moved out of opentelemetry-specification.
- v1.12.0rc2-0.32b0 release tag: the published tag on
  opentelemetry-python is v1.12.0rc2, not v1.12.0rc2-0.32b0.
- pull/227 entry: drop stray bracket inside the URL.
- #1033 link: restore missing slash between the org and repo.

* Apply suggestion from @xrmx

* docs: address review feedback on CHANGELOG links

- aws.ecs.* link: add missing closing paren so the markdown link
  terminates before " and [aws.logs.*]...".
- #1033: remove the stray "## [#1033](...)" header and fold the issue
  reference into the "Added support for PyPy3" bullet above it, matching
  the parenthesized format used elsewhere in the file.

Assisted-by: Claude Opus 4.7

---------

Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>