[terraform-ibm-icd-mongodb] Update wording in fscloud related documentation #476
Description
ICD MongoDB is FSCloud validated, so we should reword what we have documented in:
- https://github.com/terraform-ibm-modules/terraform-ibm-icd-mongodb/blob/main/modules/fscloud/README.md
- https://github.com/terraform-ibm-modules/terraform-ibm-icd-mongodb/blob/main/examples/fscloud/README.md
(Should be consistent with the wording in terraform-ibm-modules/terraform-ibm-icd-postgresql#574)