Fix "Centos" to official notation "CentOS".

Change-Id: I2f76fe3a19cd2ebf004d0a7c14852f20f3dadd9e
diff --git a/doc/source/overview.rst b/doc/source/overview.rst
index c07a8e6..814a2b1 100644
--- a/doc/source/overview.rst
+++ b/doc/source/overview.rst
@@ -24,7 +24,7 @@
 
 -  Ubuntu: current LTS release plus current development release
 -  Fedora: current release plus previous release
--  RHEL/Centos: current major release
+-  RHEL/CentOS: current major release
 -  Other OS platforms may continue to be included but the maintenance of
    those platforms shall not be assumed simply due to their presence.
    Having a listed point-of-contact for each additional OS will greatly