fix: correct DeepAcyclicGraph import path in DAG docs

The Usage section showed `from deepeval.dag import DeepAcyclicGraph`
which raises ModuleNotFoundError. The correct import is
`from deepeval.metrics.dag import DeepAcyclicGraph`, consistent with
all other examples in the same file.

Fixes #2640

Author: JiaEnChua

docs/content/docs/(custom)/metrics-dag.mdx

@@ -62,7 +62,7 @@ You'll also need to supply any additional arguments such as `expected_output` an
 The `DAGMetric` can be used to evaluate single-turn LLM interactions based on LLM-as-a-judge decision-trees.
 
 ```python
-from deepeval.dag import DeepAcyclicGraph
+from deepeval.metrics.dag import DeepAcyclicGraph
 from deepeval.metrics import DAGMetric
 
 dag = DeepAcyclicGraph(root_nodes=[...])