Fix parameter type in ml.get_categories YAML test

[pquentin]

category_id is a long, not a string:

elasticsearch/x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/rest/results/RestGetCategoriesAction.java

Lines 53 to 55 in f86f58f

	Long categoryId = restRequest.hasParam(CATEGORY_ID.getPreferredName())
	? Long.parseLong(restRequest.param(CATEGORY_ID.getPreferredName()))
	: null;

We noticed this thanks to the Elasticsearch specification CI which validates YAML tests against the Elasticsearch specification types.

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)