Using /etc/ as an example directory is a banana peel

### Issue Description
As seen [here](https://groups.google.com/g/vault-tool/c/IVYLA3aH72M), it is easy to slip up with /etc/ being a symlink in MacOS. This results in the error `can not execute files outside of configured plugin directory`.

Although an understanding of the plugin system is a prerequisite, I still found this gotcha unexpected for an example/tutorial.

## Solutions
I worked around this by setting a config.hcl with `plugin_directory = "/private/etc/vault/plugins"` and starting Vault with `vault server -config /path/to/config.hcl`

Perhaps a different directory could be used, or the README should have a warning?