fix(controller): call correct service method

Author: jaredcnance

src/JsonApiDotNetCore/Controllers/JsonApiController.cs

@@ -64,7 +64,7 @@ public virtual async Task<IActionResult> GetAsync(TId id)
         [HttpGet("{id}/relationships/{relationshipName}")]
         public virtual async Task<IActionResult> GetRelationshipsAsync(TId id, string relationshipName)
         {
-            var relationship = _resourceService.GetRelationshipAsync(id, relationshipName);
+            var relationship = _resourceService.GetRelationshipsAsync(id, relationshipName);
             if(relationship == null) 
                 return NotFound();