[bolt][Docs] fix missing option in cmake of stage3 in OptimizingClang.md (#93684)

Fixes #93681

Author: Itis-hard2name

bolt/docs/OptimizingClang.md

@@ -49,6 +49,7 @@ $ cd ${TOPLEV}/stage3
 $ CPATH=${TOPLEV}/stage2-prof-use-lto/install/bin/
 $ cmake -G Ninja ${TOPLEV}/llvm -DLLVM_TARGETS_TO_BUILD=X86 -DCMAKE_BUILD_TYPE=Release \
     -DCMAKE_C_COMPILER=$CPATH/clang -DCMAKE_CXX_COMPILER=$CPATH/clang++ \
+    -DLLVM_ENABLE_PROJECTS="clang" \
     -DLLVM_USE_LINKER=lld -DCMAKE_INSTALL_PREFIX=${TOPLEV}/stage3/install
 $ perf record -e cycles:u -j any,u -- ninja clang
 ```