Replace deprecated 'context' condition annotation with 'context_definitions' (#925)

There was a cleanup done for this deprecation in https://github.com/Islandora/islandora/pull/764
before the 'NodeReferencedByNode' condition was made, then the deprecated annotation must have
just been missed in reviewing https://github.com/Islandora/islandora/pull/808.

src/Plugin/Condition/NodeReferencedByNode.php

@@ -16,7 +16,7 @@ use Symfony\Component\DependencyInjection\ContainerInterface;
  * @Condition(
  *   id = "node_referenced_by_node",
  *   label = @Translation("Node is referenced by other nodes"),
- *   context = {
+ *   context_definitions = {
  *     "node" = @ContextDefinition("entity:node", required = TRUE , label = @Translation("node"))
  *   }
  * )