[llvm] Fix GetElementPtr article link (PR #151868)

[via llvm-commits]

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@@ -138,7 +138,7 @@ facilities for lexical nor semantic analysis.
 
 I don't understand the ``GetElementPtr`` instruction. Help!
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-See `The Often Misunderstood GEP Instruction <GetElementPtr.html>`_.
+See `The Often Misunderstood GEP Instruction <GetElementPtr.rst>`_.
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kirindenle wrote:

Oh, I see, thank you!

https://github.com/llvm/llvm-project/pull/151868