[DOCS] Fixes usage of xpack-ref attribute

[lcawl]

Summary

This PR fixes links that are incorrectly sending users to the X-Pack Reference or Stack Overview.

It also fixes the following links that will break in elastic/docs#1490

12:47:37 INFO:build_docs: /tmp/docsbuild/target_repo/html/en/kibana/6.3/install.html:
12:47:37 INFO:build_docs: - en/x-pack/6.2/xpack-security.html
12:47:37 INFO:build_docs: /tmp/docsbuild/target_repo/html/en/kibana/6.4/development-security-rbac.html:
12:47:37 INFO:build_docs: - en/x-pack/6.2/security-privileges.html#application-privileges
12:47:37 INFO:build_docs: /tmp/docsbuild/target_repo/html/en/kibana/6.4/install.html:
12:47:37 INFO:build_docs: - en/x-pack/6.2/xpack-security.html
12:47:37 INFO:build_docs: /tmp/docsbuild/target_repo/html/en/kibana/6.5/development-security-rbac.html:
12:47:37 INFO:build_docs: - en/x-pack/6.2/security-privileges.html#application-privileges
12:47:37 INFO:build_docs: /tmp/docsbuild/target_repo/html/en/kibana/6.5/install.html:
12:47:37 INFO:build_docs: - en/x-pack/6.2/xpack-security.html
12:47:37 INFO:build_docs: /tmp/docsbuild/target_repo/html/en/kibana/6.6/development-security-rbac.html:
12:47:37 INFO:build_docs: - en/x-pack/6.2/security-privileges.html#application-privileges
12:47:37 INFO:build_docs: /tmp/docsbuild/target_repo/html/en/kibana/6.6/install.html:
12:47:37 INFO:build_docs: - en/x-pack/6.2/xpack-security.html
12:47:37 INFO:build_docs: /tmp/docsbuild/target_repo/html/en/kibana/6.7/development-security-rbac.html:
12:47:37 INFO:build_docs: - en/x-pack/6.2/security-privileges.html#application-privileges
12:47:37 INFO:build_docs: /tmp/docsbuild/target_repo/html/en/kibana/6.7/install.html:
12:47:37 INFO:build_docs: - en/x-pack/6.2/xpack-security.html
12:47:37 INFO:build_docs: /tmp/docsbuild/target_repo/html/en/kibana/7.0/development-security-rbac.html:
12:47:37 INFO:build_docs: - en/x-pack/6.2/security-privileges.html#application-privileges
12:47:37 INFO:build_docs: /tmp/docsbuild/target_repo/html/en/kibana/7.1/development-security-rbac.html:
12:47:37 INFO:build_docs: - en/x-pack/6.2/security-privileges.html#application-privileges
12:47:37 INFO:build_docs: /tmp/docsbuild/target_repo/html/en/kibana/7.2/development-security-rbac.html:
12:47:37 INFO:build_docs: - en/x-pack/6.2/security-privileges.html#application-privileges

[skip ci]

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

• This was checked for cross-browser compatibility, including a check against IE11
• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

[elasticmachine]

Pinging @elastic/kibana-docs (Team:Docs)