[clang] [clang/www/get_started.html] Use newer `cmake` syntax (PR #93503)

[Samuel Marks via cfe-commits]

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@@ -74,8 +74,8 @@ <h3 id="buildNix">On Unix-like Systems</h3>
         <a href="https://llvm.org/docs/CMake.html#frequently-used-cmake-variables">frequently used cmake variables</a>
         for more options.
     </li>
-    <li><tt>cmake -DLLVM_ENABLE_PROJECTS=clang -DCMAKE_BUILD_TYPE=Release -G "Unix Makefiles" ../llvm</tt></li>
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SamuelMarks wrote:

Oh I thought someone told me to use Ninja here in the docs I can revert. It's already referenced for Windows though - https://github.com/llvm/llvm-project/blob/12b0ef5dade664fb24cb4e627f1c391d937c0925/clang/www/get_started.html#L216 - still want the revert?

https://github.com/llvm/llvm-project/pull/93503