Fixed typo in test header file comment.

include/SDL_test.h

@@ -52,7 +52,7 @@ extern "C" {
 /* Global definitions */
 
 /*
- * Note: Maximum size of SDLTest log message is less than SDLs limit
+ * Note: Maximum size of SDLTest log message is less than SDL's limit
  * to ensure we can fit additional information such as the timestamp.
  */
 #define SDLTEST_MAX_LOGMESSAGE_LENGTH   3584