fix: correct headings levels for Previous Releases page (#6682)

Author: theoludwig

pages/en/about/previous-releases.mdx

@@ -10,12 +10,12 @@ After six months, odd-numbered releases (9, 11, etc.) become unsupported, and ev
 _LTS_ release status is "long-term support", which typically guarantees that critical bugs will be fixed for a total of 30 months.
 Production applications should only use _Active LTS_ or _Maintenance LTS_ releases.
 
-### Release Schedule
+## Release Schedule
 
 ![Releases](https://raw.githubusercontent.com/nodejs/Release/main/schedule.svg?sanitize=true)
 
 Full details regarding Node.js release schedule are available [on GitHub](https://github.com/nodejs/release#release-schedule).
 
-### Looking for latest release of a version branch?
+## Looking for latest release of a version branch?
 
 <DownloadReleasesTable />