Update policy example in the documentation

Update the policy example in the policies documentation to use the
latest version of the policy definition for the visibility setting of
GitHub repositories.

Resolves #422

docs/policies.md

@@ -62,14 +62,15 @@ $ terrascan scan -i terraform --config-only -o json
 }
 ```
 
-You can use this `.json` output as the input in the (rego playgound)[https://play.openpolicyagent.org/]. The following policy can be used on the above Terraform to flag if the GitHub repository has been created with `private = false`.
+You can use this `.json` output as the input in the (rego playgound)[https://play.openpolicyagent.org/]. The following policy can be used on the above Terraform to flag if the GitHub repository has been created with `private = false` or `visibility = public`, depending on the version used for the GitHub provider.
 
 ```
 package accurics
 
 privateRepoEnabled[api.id] {
-api := input.github_repository[_]
-not api.config.private == true
+    api := input.github_repository[_]
+    not api.config.private == true
+    not api.config.visibility == "private"
 }
 ```