Fix typo in `configuration_gpt2.py` (#25676)

Update configuration_gpt2.py

src/transformers/models/gpt2/configuration_gpt2.py

@@ -60,7 +60,7 @@ class GPT2Config(PretrainedConfig):
             Number of attention heads for each attention layer in the Transformer encoder.
         n_inner (`int`, *optional*, defaults to None):
             Dimensionality of the inner feed-forward layers. `None` will set it to 4 times n_embd
-        activation_function (`str`, *optional*, defaults to `"gelu"`):
+        activation_function (`str`, *optional*, defaults to `"gelu_new"`):
             Activation function, to be selected in the list `["relu", "silu", "gelu", "tanh", "gelu_new"]`.
         resid_pdrop (`float`, *optional*, defaults to 0.1):
             The dropout probability for all fully connected layers in the embeddings, encoder, and pooler.