[examples] rename example label

appl/example1/CMakeLists.txt

 frackit_add_application(NAME example1
                         SOURCES example1.cc
-                        LABELS examples)
+                        LABELS example)
 
 set(CMAKE_BUILD_TYPE Release)

appl/example2/CMakeLists.txt

 frackit_add_application(NAME example2
                         SOURCES example2.cc
-                        LABELS examples)
+                        LABELS example)
 
 set(CMAKE_BUILD_TYPE Release)

appl/example3/CMakeLists.txt

 frackit_add_application(NAME example3
                         SOURCES example3.cc
                         COMPILE_DEFINITIONS BREPFILE="${CMAKE_SOURCE_DIR}/appl/example3/layers.brep"
-                        LABELS examples)
+                        LABELS example)
 
 set(CMAKE_BUILD_TYPE Release)