[BUG] Fix Cloud Native Security ref link

[jmikell821]

Documentation links

https://www.elastic.co/guide/en/security/current/cloud-native-security-overview.html

Description

In the CSPM section, the reference at the end says, "To learn more, refer to Read the CSPM docs." We need to fix the link text.

Which documentation set(s) does this bug apply to?

ESS and serverless

Release version

Really all versions for which this page existed, but we can do 8.16 (8x), 8.15 and 8.14.

Testing environment

Any test environment will do.