[V1][Bugfix] Fix typo in MoE TPU checking (#15927)

Signed-off-by: Roger Wang <ywang@roblox.com>

Author: ywang96

vllm/model_executor/layers/fused_moe/layer.py

@@ -309,7 +309,7 @@ def forward_tpu(
                                 expert_map=expert_map,
                                 renormalize=renormalize)
 
-    forward_native = forward_tpu if current_platform.is_tpu else forward_cuda
+    forward_native = forward_tpu if current_platform.is_tpu() else forward_cuda
 
 
 def determine_expert_map(