The example in the documentation correctly passes a quoted boolean (i.e.
true or false as a string) instead of a "real" HCL boolean. This commit
corrects the parameter list to document that fact.
While it would be more desirable to change the implementation to accept
an unquoted boolean, it seems that the use of `hcl.DecodeObject` for
parameters which are not common to all storage back ends would make this
a rather more involved change than this necessarily warrants.
With `ha_enabled = true` vault crashes with the following error:
```
error parsing 'storage': storage.dynamodb: At 17:16: root.ha_enabled: unknown type for string *ast.LiteralType
```
This seems related to https://github.com/hashicorp/vault/issues/1559
