doc: Minor formatting enhancement

The security advisories would all appear on a single line. Use bullet
points instead to improve the readability.

Change-Id: Id631985d7d559b3632f43d695cffa6735520b64a
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

readme.rst

@@ -263,14 +263,14 @@ Arm licensees may contact Arm directly via their partner managers.
 Security advisories
 ~~~~~~~~~~~~~~~~~~~
 
-`Security Advisory TFV-1`_
-`Security Advisory TFV-2`_
-`Security Advisory TFV-3`_
-`Security Advisory TFV-4`_
-`Security Advisory TFV-5`_
-`Security Advisory TFV-6`_
-`Security Advisory TFV-7`_
-`Security Advisory TFV-8`_
+-  `Security Advisory TFV-1`_
+-  `Security Advisory TFV-2`_
+-  `Security Advisory TFV-3`_
+-  `Security Advisory TFV-4`_
+-  `Security Advisory TFV-5`_
+-  `Security Advisory TFV-6`_
+-  `Security Advisory TFV-7`_
+-  `Security Advisory TFV-8`_
 
 
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