Testing/External: fix the module path

Testing/External/CMakeLists.txt

@@ -18,7 +18,7 @@ set(VTK_SOURCE_DIR "${VTK_TOP_DIR}")
 # One should provide VTK_DIR explicitly in our intended use cases.
 find_package(VTK REQUIRED NO_MODULE)
 # Tests use some macros only stored in VTK's source tree.
-list(APPEND CMAKE_MODULE_PATH "${VTK_TOP_DIR}/cmake")
+list(APPEND CMAKE_MODULE_PATH "${VTK_TOP_DIR}/CMake")
 
 # This is a cross-platform project so we cannot use the MS _s API.
 if(WIN32 AND CMAKE_C_COMPILER_ID STREQUAL "Intel")