[clang] [clang][docs] Fix the __block import example in the Block ABI (PR #226626)

[Akira Hatanaka via cfe-commits]

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ahatanak wrote:

The struct type should be `_block_byref_obj`. `_block_byref_voidBlock` doesn't have field `captured_obj`.

https://github.com/llvm/llvm-project/pull/226626