CMakeLists: lowercase find_library usage

The rest of the CMake script uses lowercase for commands (which is the
general CMake style), making it more consistent with surrounding code.

CMakeLists.txt

@@ -303,7 +303,7 @@ endif()
 # ======================================
 
 IF (APPLE)
-    FIND_LIBRARY(COCOA_LIBRARY Cocoa)           # Umbrella framework for everything GUI-related
+    find_library(COCOA_LIBRARY Cocoa)           # Umbrella framework for everything GUI-related
     set(PLATFORM_LIBRARIES ${COCOA_LIBRARY} ${IOKIT_LIBRARY} ${COREVIDEO_LIBRARY})
 
     if (CMAKE_CXX_COMPILER_ID STREQUAL Clang)