chore(docs): Make plan explicit (#535)

Keys created by this module will only be found by the UI or CLI if a Standard plan instance is ID is provided. Universal Key Orchestrator (UKO) plan instances will not be able to find/display keys.

Co-authored-by: Conall Ó Cofaigh <ocofaigh@ie.ibm.com>

README.md

@@ -6,7 +6,7 @@
 [![latest release](https://img.shields.io/github/v/release/terraform-ibm-modules/terraform-ibm-kms-key?logo=GitHub&sort=semver)](https://github.com/terraform-ibm-modules/terraform-ibm-kms-key/releases/latest)
 [![Renovate enabled](https://img.shields.io/badge/renovate-enabled-brightgreen.svg)](https://renovatebot.com/)
 
-This module supports creating a standard or root key in an existing key ring and KMS instance. KMS can be Key Protect or Hyper Protect Crypto Services(HPCS) instance. You can specify rotation and deletion policies for the keys.
+This module supports creating a standard or root key in an existing key ring and KMS instance. KMS can be Key Protect or Hyper Protect Crypto Services (HPCS) Standard plan instance. You can specify rotation and deletion policies for the keys.
 
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