[doc] [cmake] Fix a typo in examples for the cmake directory docs. NFC.

[mstorsjo]

Summary

The previous case was a tautology - this is probably what was intended.

[phosek]

LGTM

[Ericson2314]

Oh hah! Thanks for catching.

cmake/README.rst

 How to use
 ----------
 
 For tools, please do:
 
 .. code-block:: cmake
 
   if(NOT DEFINED LLVM_COMMON_CMAKE_UTILS)
-    set(LLVM_COMMON_CMAKE_UTILS ${LLVM_COMMON_CMAKE_UTILS}/../cmake)
+    set(LLVM_COMMON_CMAKE_UTILS ${CMAKE_CURRENT_SOURCE_DIR}/../cmake)
   endif()
 
   # Add path for custom modules.
   list(INSERT CMAKE_MODULE_PATH 0
     # project-specific module dirs first
     "${LLVM_COMMON_CMAKE_UTILS}/Modules"
     )
 
 Notes:
 
 - The ``if(NOT DEFINED ...)`` guard is there because in combined builds, LLVM
   will set this variable.  This is useful for legacy builds where projects are
   found in ``llvm/tools`` instead.
 
 - ``INSERT ... 0`` ensures these new entries are prepended to the front of the
   module path, so nothing might shadow them by mistake.
 
 For runtime libs, we skip the ``if(NOT DEFINED`` part:
 
 .. code-block:: cmake
 
-  set(LLVM_COMMON_CMAKE_UTILS ${LLVM_COMMON_CMAKE_UTILS}/../cmake)
+  set(LLVM_COMMON_CMAKE_UTILS ${CMAKE_CURRENT_SOURCE_DIR}/../cmake)
 
   ... # same as before
 
 If ``llvm/tools`` legacy-style combined builds are deprecated, we should then
 skip it everywhere, bringing the tools and runtimes boilerplate back in line.