Change discovery_agent interface name

Interface '\core\hook\hook_discovery_agent' does not exist in the \core\hook. It should be '\core\hook\discovery_agent' as described above the code example.

Author: volodymyrdovhan

versioned_docs/version-4.5/apis/core/hooks/index.md

@@ -126,7 +126,7 @@ If you define a hook which is _not_ in the `[component]\hook\*` namespace then y
 
 namespace mod_example;
 
-class hooks implements \core\hook\hook_discovery_agent {
+class hooks implements \core\hook\discovery_agent {
     public static function discover_hooks(): array {
         return [
             [