Fix GettingStarted doc so that the example build command for cmake LL…

…VM_ENABLE_PROJECTS works on linux

I tested the previous one on macOS, however building libc++ on Linux
requires libcxxabi as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292290 91177308-0d34-0410-b5e6-96231b3b80d8

docs/GettingStarted.rst

@@ -725,7 +725,7 @@ Or a combination of multiple projects:
 
   % cd $TOP_LEVEL_DIR
   % mkdir clang-build && cd clang-build
-  % cmake -GNinja ../llvm-project/llvm -DLLVM_ENABLE_PROJECTS="clang;libcxx;compiler-rt"
+  % cmake -GNinja ../llvm-project/llvm -DLLVM_ENABLE_PROJECTS="clang;libcxx;libcxxabi"
 
 A helper script is provided in `llvm/utils/git-svn/git-llvm`. After you add it
 to your path, you can push committed changes upstream with `git llvm push`.