Fix 'default' in usage documentation

[rsenden]

The code snippet in https://github.com/fortify/tool-definitions/blob/main/doc-resources/repo-usage.md#tool-definition-v1-file-structure still mentions 'default' as a supported type/platform.

This needs to be updated as fcli will no longer look for a 'default' entry. Instead, the various tool-specific install commands now implement a getDefaultArtifactType() method that returns the default artifact type for that particular tool (which for most tools is java).