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

[Vlad Serebrennikov 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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Endilll wrote:

Sorry, I don't think we can tell people to use ninja here as a matter of fact, because it's not in our requirements documented here: https://llvm.org/docs/GettingStarted.html#software. We still can suggest it somewhere if it's available.

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