[libc++][Docs] Update benchmark doc wrt monorepo

Seems this section is not updated since we have transited to llvm-project monorepo.
At the start, we build libcxx under monorepo configuration but later try to make the separate configuration for libcxx build
and running benchmark.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D110722

libcxx/docs/TestingLibcxx.rst

@@ -227,12 +227,11 @@ An example build would look like:
 .. code-block:: bash
 
   $ cd build
-  $ cmake [options] <path to libcxx sources>
-  $ make cxx-benchmarks
+  $ ninja cxx-benchmarks
 
 This will build all of the benchmarks under ``<libcxx-src>/benchmarks`` to be
 built against the just-built libc++. The compiled tests are output into
-``build/benchmarks``.
+``build/projects/libcxx/benchmarks``.
 
 The benchmarks can also be built against the platforms native standard library
 using the ``-DLIBCXX_BUILD_BENCHMARKS_NATIVE_STDLIB=ON`` CMake option. This
@@ -255,8 +254,7 @@ For example:
 
 .. code-block:: bash
 
-  $ cd build/benchmarks
-  $ make cxx-benchmarks
+  $ cd build/projects/libcxx/benchmarks
   $ ./algorithms.libcxx.out # Runs all the benchmarks
   $ ./algorithms.libcxx.out --benchmark_filter=BM_Sort.* # Only runs the sort benchmarks