Update ex5 makefile

ex5/Makefile

 LIB=-lm
 CC=gcc -Wall -Wextra -pedantic -g
 
-ex4: ex5.o
+ex5: ex5.o
 	$(CC) $^ -fsanitize=address -fsanitize=leak -o $@ $(LIB)
 
-ex4.o: ex5.c
+ex5.o: ex5.c
 	$(CC) -c $< $(LIB)
 clean:
 	rm -f *.o ex5