libcxx: fix a documentation typo

See `grep 'option.LIBCXX_INCLUDE_TESTS' libcxx/CMakeLists.txt`.

GitOrigin-RevId: 9d36f70ef28a1e6bba763b40c9c49a76c520a56e
diff --git a/docs/BuildingLibcxx.rst b/docs/BuildingLibcxx.rst
index 67ce03d..da72aac 100644
--- a/docs/BuildingLibcxx.rst
+++ b/docs/BuildingLibcxx.rst
@@ -292,7 +292,7 @@
 
 .. option:: LIBCXX_INCLUDE_TESTS:BOOL
 
-  **Default**: ``ON`` (or value of ``LLVM_INCLUDE_DIR``)
+  **Default**: ``ON`` (or value of ``LLVM_INCLUDE_TESTS``)
 
   Build the libc++ tests.