fix: use valid Keep a Changelog subsection type 'Changed' instead of 'Performance'

'### Performance' is not a recognised Keep a Changelog subsection.
Ionide.KeepAChangelog.Tasks 0.3.3 enforces the standard categories
(Added, Changed, Deprecated, Fixed, Removed, Security), causing the
build to fail with IKC0002. Rename to '### Changed'.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

RELEASE_NOTES.md

@@ -8,7 +8,7 @@
 * Fix `Markdown.ToMd` serialising italic spans with asterisks incorrectly as bold spans. [#1102](https://github.com/fsprojects/FSharp.Formatting/pull/1102)
 * Fix `Markdown.ToMd` serialising ordered list items with incorrect numbering and formatting. [#1102](https://github.com/fsprojects/FSharp.Formatting/pull/1102)
 
-### Performance
+### Changed
 * `fsdocs build` now pre-computes the navigation menu structure (filter/group/sort) once per build rather than once per output page, reducing work from O(n²) to O(n) for sites with n pages. The filesystem check for custom menu templates is also cached per build. [#1129](https://github.com/fsprojects/FSharp.Formatting/pull/1129)
 
 ## [22.0.0-alpha.2] - 2026-03-13