doc: Fix a broken link in the readme.rst file

Change-Id: I53a4649b17614f711957424ddffed1dcccfc7880
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

readme.rst

@@ -255,7 +255,7 @@ Feedback and support
 
 Arm welcomes any feedback on TF-A. If you think you have found a security
 vulnerability, please report this using the process defined in the TF-A
-`Security Centre`_. For all other feedback, please use the
+`Security Center`_. For all other feedback, please use the
 `GitHub issue tracker`_.
 
 Arm licensees may contact Arm directly via their partner managers.