Update 3_2_release_notes.md

Fixed typo and applied proper code highlighting.
Missing destroy command
Fix typos: the indefinite articles(a -> an)
Merge pull request #12216 from suginoy/a-an

Fix typos: the indefinite articles(a -> an).
Merge pull request #12215 from harshadsabne/master

Update 3_2_release_notes.md
Custom flash should be defined only for the class that defines it and it's subclasses.
Merge pull request #12222 from rcillo/fix-custom-flash

Custom flash should be defined only for the class that defines it and it's subclasses

Fixes #12057