[bolt][Docs] execute ninja clang failed in stage3

[Itis-hard2name]

in the "Optimizing Clang with BOLT" section of bolt/docs/OptimizingClang.md
when executing perf record -e cycles:u -j any,u -- ninja clang,
it fails and the following message is displayed:
ninja: error: unknown target 'clang', did you mean 'clean'?
The reason for the failure is that -DLLVM_ENABLE_PROJECTS="clang" is not added

# The original command
$ cmake -G Ninja ${TOPLEV}/llvm -DLLVM_TARGETS_TO_BUILD=X86 -DCMAKE_BUILD_TYPE=Release \
    -DCMAKE_C_COMPILER=$CPATH/clang -DCMAKE_CXX_COMPILER=$CPATH/clang++ \
    -DLLVM_USE_LINKER=lld -DCMAKE_INSTALL_PREFIX=${TOPLEV}/stage3/install