Update TestingLibcxx doc to reflect the use_system_cxx_lib flag. Patch from Michael Park

llvm-svn: 290441
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: aaec00aed5a8963ba6f421c9620332228352f4b0
diff --git a/docs/TestingLibcxx.rst b/docs/TestingLibcxx.rst
index 651d730..e429276 100644
--- a/docs/TestingLibcxx.rst
+++ b/docs/TestingLibcxx.rst
@@ -130,7 +130,7 @@
 
   Specify the directory of the libc++ library to be tested. By default the
   library folder of the build directory is used. This option cannot be used
-  when use_system_lib is provided.
+  when use_system_cxx_lib is provided.
 
 
 .. option:: cxx_runtime_root=<path/to/lib/>
@@ -139,9 +139,9 @@
   is not added to the linkers search path. This can be used to compile tests
   against one version of libc++ and run them using another. The default value
   for this option is `cxx_library_root`. This option cannot be used
-  when use_system_lib is provided.
+  when use_system_cxx_lib is provided.
 
-.. option:: use_system_lib=<bool>
+.. option:: use_system_cxx_lib=<bool>
 
   **Default**: False