[libc++][docs] Use --show-all in the sample command-line to run benchmarks

It's really not useful at all to run benchmarks without --show-all since
you don't get the benchmark output. And since --show-all is the suggested
default way to run benchmarks, it's not necessary anymore to mention it
right below.

Author: ldionne

libcxx/docs/TestingLibcxx.rst

@@ -452,12 +452,7 @@ when running the benchmarks. For example,
 
 .. code-block:: bash
 
-  $ libcxx/utils/libcxx-lit <build> -sv libcxx/test/benchmarks/string.bench.cpp --param optimization=speed
-
-If you want to see where a benchmark is located (e.g. you want to store the executable
-for subsequent analysis), you can print that information by passing ``--show-all`` to
-``lit``. That will print the command-lines being executed, which includes the location
-of the executable created for that benchmark.
+  $ libcxx/utils/libcxx-lit <build> libcxx/test/benchmarks/string.bench.cpp --show-all --param optimization=speed
 
 Note that benchmarks are only dry-run when run via the ``check-cxx`` target since
 we only want to make sure they don't rot. Do not rely on the results of benchmarks