Update server version from 2.17 to 2.18

This page was under the heading for 2.18 server installation

Author: annapamma

jekyll/_cci2/aws.adoc

@@ -6,7 +6,7 @@
 :toc: macro
 :toc-title:
 
-Following is a step by step guide to installing CircleCI Server v2.17 with Terraform.
+Following is a step by step guide to installing CircleCI Server v2.18 with Terraform.
 
 toc::[]