RELEASE_NOTES: Properly encapsulate symbols in backticks

This allows the output to look nicer, and makes markdown less
confused when it finds an `_`.

Change-Id: Ib335728a2d256826a45e2e2fa3174e0a004e9fab
Reviewed-on: http://review.couchbase.org/39278
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>