docs: Use proper return type for custom template listing

[juliusknorr]

Came up with nextcloud/richdocuments#4386 in psalm

We have the wrong return type annotated, the returned list is using Template objects, which is also what all existing implementations do and what the usage expects:

server/lib/private/Files/Template/TemplateManager.php

Lines 188 to 195 in efa615b

	* @return list<Template>
	*/
	private function getTemplateFiles(TemplateFileCreator $type): array {
	$templates = [];
	foreach ($this->getRegisteredProviders() as $provider) {
	foreach ($type->getMimetypes() as $mimetype) {
	foreach ($provider->getCustomTemplates($mimetype) as $template) {
	$templates[] = $template;

https://github.com/nextcloud/richdocuments/blob/ba9c364e952fed85dd7431f711b57246b9e268f3/lib/Template/CollaboraTemplateProvider.php#L47
https://github.com/ONLYOFFICE/onlyoffice-nextcloud/blob/d20b985948957de6785a5e126aa9d63dd668b453/lib/TemplateProvider.php#L69