[mlir][bufferization] fix typo in example code for bufferization.alloc_tensor

See BufferizationOps.cpp:408, the dynamic sizes are enclosed by "()" not "[]"

https://github.com/llvm/llvm-project/blob/main/mlir/lib/Dialect/Bufferization/IR/BufferizationOps.cpp#L408

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D133505