Fix a link in sgl-kernel/README.md (#5493)

sgl-kernel/README.md

@@ -47,7 +47,7 @@ Steps to add a new kernel:
 
 1. Implement the kernel in [csrc](https://github.com/sgl-project/sglang/tree/main/sgl-kernel/csrc)
 2. Expose the interface in [include/sgl_kernel_ops.h](https://github.com/sgl-project/sglang/blob/main/sgl-kernel/include/sgl_kernel_ops.h)
-3. Create torch extension in [csrc/torch_extension.cc](https://github.com/sgl-project/sglang/blob/main/sgl-kernel/csrc/torch_extension.cc)
+3. Create torch extension in [csrc/common_extension.cc](https://github.com/sgl-project/sglang/blob/main/sgl-kernel/csrc/common_extension.cc)
 4. Update [CMakeLists.txt](https://github.com/sgl-project/sglang/blob/main/sgl-kernel/CMakeLists.txt) to include new CUDA source
 5. Expose Python interface in [python](https://github.com/sgl-project/sglang/blob/main/sgl-kernel/python/sgl_kernel)