HowToBuildWithPGO.rst: Fix a few details in the manual steps

[hans]

Summary

The steps were easy to follow, I just noticed a few details to adjust.

[george.burgess.iv]

Thanks!

(Also thanks for the reminder that I still need to update this with commentary for the purely-cmake-based approach. :) )

docs/HowToBuildWithPGO.rst

a. Build your benchmark using the Clang generated in step 2. The 'standard'
       ``-DCMAKE_BUILD_TYPE=RelWithDebInfo`` instead of
       ``-DCMAKE_BUILD_TYPE=Release``. This will grant better coverage of
       debug info pieces of clang, but will take longer to complete and will
       result in a much larger build directory.
 
       It's recommended to build the ``all`` target with your instrumented Clang,
       since more coverage is often better.
 
-  b. You should now have a few ``*.profdata`` files in
+  b. You should now have a few ``*.profraw`` files in
      ``path/to/stage2/profiles/``. You need to merge these using
      ``llvm-profdata`` (even if you only have one! The profile merge transforms
      profraw into actual profile data, as well). This can be done with
-     ``/path/to/stage1/llvm-profdata -merge
-     -output=/path/to/output/profdata.prof path/to/stage2/profiles/*.profdata``.
+     ``/path/to/stage1/llvm-profdata merge
+     -output=/path/to/output/profdata.prof path/to/stage2/profiles/*.profraw``.
 
 4. Now, build your final, PGO-optimized Clang. To do this, you'll want to pass
    the following additional arguments to CMake.
 
    - ``-DLLVM_PROFDATA_FILE=/path/to/output/profdata.prof`` - Use the PGO
      profile from the previous step.
    - ``-DCMAKE_C_COMPILER=/path/to/stage1/clang`` - Use the Clang we built in
      step 1.