Fixed comment (thanks @pionere!)

src/SDL_utils_c.h

@@ -24,7 +24,7 @@
 
 /* Common utility functions that aren't in the public API */
 
-/* Return the smallest power of 2 greater than or equal to 2 */
+/* Return the smallest power of 2 greater than or equal to 'x' */
 int SDL_powerof2(int x);
 
 #endif /* SDL_utils_h_ */