Fixing prop-type for FormTemplate

Fixes error:
```
Warning: Failed prop type: Invalid prop `FormTemplate` of type `object` supplied to `FormRenderer`, expected `function`.
```

Author: zgover

packages/react-form-renderer/src/form-renderer/form-renderer.js

@@ -103,7 +103,7 @@ FormRenderer.propTypes = {
   componentMapper: PropTypes.shape({
     [PropTypes.string]: PropTypes.oneOfType([PropTypes.node, PropTypes.element, PropTypes.func])
   }).isRequired,
-  FormTemplate: PropTypes.func.isRequired,
+  FormTemplate: PropTypes.elementType.isRequired,
   validatorMapper: PropTypes.shape({
     [PropTypes.string]: PropTypes.func
   }),