[Threading] Fix comment.

I changed the directory name but not the comment here.

rdar://99125585

Author: al45tair

lib/Threading/CMakeLists.txt

@@ -1,5 +1,5 @@
 # If you update this, you also need to update the CMakeLists.txt file in
-# stdlib/public/Threading AND the one in stdlib/toolchain/Threading
+# stdlib/public/Threading AND the one in stdlib/toolchain/CompatibilityThreading
 
 # Note that it is *not* an error that Errors.cpp is only listed here.
 # It shouldn't be in the other places because those are OBJECT_LIBRARYs