misc: Fix typo in RELEASE-NOTES.md
misc: Fix typo in RELEASE-NOTES.md

"21.0.1" -> "21.2.1".

Change-Id: Ia5d7780015bf803931e393d0716cafb51ac7a7d2

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5