demos/CMakeLists.txt: make header files belong to their respective demo

This enables IDEs to show them as part of the project. The code to do it
was already there, but it was broken.

demos/CMakeLists.txt

@@ -159,13 +159,13 @@ macro(ie_add_sample)
 
     # Create named folders for the sources within the .vcproj
     # Empty name lists them directly under the .vcproj
-    source_group("src" FILES ${IE_SAMPLES_SOURCES})
-    if(IE_SAMPLES_HEADERS)
-        source_group("include" FILES ${IE_SAMPLES_HEADERS})
+    source_group("src" FILES ${IE_SAMPLE_SOURCES})
+    if(IE_SAMPLE_HEADERS)
+        source_group("include" FILES ${IE_SAMPLE_HEADERS})
     endif()
 
     # Create executable file from sources
-    add_executable(${IE_SAMPLE_NAME} ${IE_SAMPLE_SOURCES} ${IE_SAMPLES_HEADERS})
+    add_executable(${IE_SAMPLE_NAME} ${IE_SAMPLE_SOURCES} ${IE_SAMPLE_HEADERS})
 
     if(WIN32)
         set_target_properties(${IE_SAMPLE_NAME} PROPERTIES COMPILE_PDB_NAME ${IE_SAMPLE_NAME})