[clang] [NFC][Doc] Fix non-existing reference in BuiltinsAMDGPUDocs.td (PR #201889)

[Shilei Tian via cfe-commits]

shiltian wrote:

* #201889 👈
* `main`

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This stack of pull requests is managed by <a href="https://github.com/shiltian/sgh"><b>sgh</b></a>.
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https://github.com/llvm/llvm-project/pull/201889