Added RHEL 7

Author: Alexander Nosov

docs/sql-server/azure-arc/overview.md

@@ -33,7 +33,7 @@ Azure Arc enabled SQL Server supports SQL Server 2012 or higher running on one o
 
 - Windows Server 2012 R2 and higher
 - Ubuntu 16.04 and 18.04 (x64)
-- CentOS Linux 7 (x64)
+- Red Hat Enterprise Linux (RHEL) 7 (x64) 
 - SUSE Linux Enterprise Server (SLES) 15 (x64)
 
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