[ThinLTO] Spell `llvm-ar` correctly.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@283217 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ThinLTO.rst

@@ -140,7 +140,7 @@ To bootstrap clang/LLVM with ThinLTO, follow these steps:
   * ``-DCMAKE_C_COMPILER=/path/to/host/clang``
   * ``-DCMAKE_CXX_COMPILER=/path/to/host/clang++``
   * ``-DCMAKE_RANLIB=/path/to/host/llvm-ranlib``
-  * ``-DCMAKE_AR=/path/to/host/llvm-nm``
+  * ``-DCMAKE_AR=/path/to/host/llvm-ar``
 
 #. To use additional linker arguments for controlling the backend
    parallelism_ or enabling incremental_ builds of the bootstrap compiler,