Fix task name error

src/deepeval/deepeval_task_manager.py

@@ -15,7 +15,7 @@ class Task(Enum):
 class DeepEvalTaskManager:
     def __init__(self, model_name, tasks: List[str]):
         self.model_name = model_name
-        self.available_tasks = {task.name: getattr(self, task.name.lower()) for task in Task}
+        self.available_tasks = {task.name: getattr(Task, task.name).value for task in Task}
         self.tasks_to_run = self.validate_tasks(tasks)
 
     def validate_tasks(self, user_tasks):