[docs] Fix include-what-you-use example

The --mapping_file switch was missing; the example would have been
rejected.

Reviewed By: #libc, Mordante

Differential Revision: https://reviews.llvm.org/D157785

Author: kimgr

libcxx/docs/UsingLibcxx.rst

@@ -133,7 +133,7 @@ IWYU, you should run the tool like so:
 
 .. code-block:: bash
 
-  $ include-what-you-use -Xiwyu /path/to/libcxx/include/libcxx.imp file.cpp
+  $ include-what-you-use -Xiwyu --mapping_file=/path/to/libcxx/include/libcxx.imp file.cpp
 
 If you would prefer to not use that flag, then you can replace ``/path/to/include-what-you-use/share/libcxx.imp``
 file with the libc++-provided ``libcxx.imp`` file.