server/tags: change response key

Since we're using "results" everywhere else, it makes little sense for
tag siblings to use "siblings" node.

API.md

@@ -491,7 +491,7 @@ data.
 
     ```json5
     {
-        "siblings": [
+        "results": [
             {
                 "tag": <tag>,
                 "occurrences": <occurrence-count>

server/szurubooru/api/tag_api.py

@@ -100,4 +100,4 @@ class TagSiblingsApi(BaseApi):
                 'tag': tags.serialize_tag(sibling),
                 'occurrences': occurrences
             })
-        return {'siblings': serialized_siblings}
+        return {'results': serialized_siblings}

server/szurubooru/tests/api/test_tag_siblings_retrieving.py

@@ -5,7 +5,7 @@ from szurubooru.func import util, tags
 
 def assert_results(result, expected_tag_names_and_occurrences):
     actual_tag_names_and_occurences = {}
-    for item in result['siblings']:
+    for item in result['results']:
         tag_name = item['tag']['names'][0]
         occurrences = item['occurrences']
         actual_tag_names_and_occurences[tag_name] = occurrences