rhel page keywords update

Signed-off-by: Lokesh Mandvekar <lsm5@redhat.com>

docs/sources/installation/rhel.rst

@@ -1,6 +1,6 @@
 :title: Requirements and Installation on Red Hat Enterprise Linux
 :description: Please note this project is currently under heavy development. It should not be used in production.
-:keywords: Docker, Docker documentation, requirements, linux, rhel
+:keywords: Docker, Docker documentation, requirements, linux, rhel, centos
 
 .. _rhel: