[chore] correct qk norm list. (huggingface#10876)

correct qk norm list.

Author: sayakpaul

src/diffusers/models/attention_processor.py

@@ -213,7 +213,9 @@ def __init__(
             self.norm_q = LpNorm(p=2, dim=-1, eps=eps)
             self.norm_k = LpNorm(p=2, dim=-1, eps=eps)
         else:
-            raise ValueError(f"unknown qk_norm: {qk_norm}. Should be None,'layer_norm','fp32_layer_norm','rms_norm'")
+            raise ValueError(
+                f"unknown qk_norm: {qk_norm}. Should be one of None, 'layer_norm', 'fp32_layer_norm', 'layer_norm_across_heads', 'rms_norm', 'rms_norm_across_heads', 'l2'."
+            )
 
         if cross_attention_norm is None:
             self.norm_cross = None