Corrected a typo in the building libc++ docs

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@319631 91177308-0d34-0410-b5e6-96231b3b80d8

docs/BuildingLibcxx.rst

@@ -130,7 +130,7 @@ just specify a toolset.
           -DCMAKE_SYSTEM_NAME=Windows                                                 ^
           -DCMAKE_C_COMPILER=clang-cl                                                 ^
           -DCMAKE_C_FLAGS="-fms-compatibility-version=19.00 --target=i686--windows"   ^
-          -DCMAKE_CXX_COMPILER=clang-c                                                ^
+          -DCMAKE_CXX_COMPILER=clang-cl                                                ^
           -DCMAKE_CXX_FLAGS="-fms-compatibility-version=19.00 --target=i686--windows" ^
           -DLLVM_PATH=/path/to/llvm/tree                                              ^
           -DLIBCXX_ENABLE_SHARED=YES                                                  ^