core: Correct an error in doxygen comments.

One of previous commits added an extra comment-closing sequence
which caused a compilation error. This patch fixes that.

timer_funcs.h

@@ -57,13 +57,12 @@ struct timer_head{
  * (1<<H0_BITS)*sizeof(struct timer_head) bytes of memory, so arrange it
  * accordingly
  *
-x* Uses ~280K on a 64 bits system and ~140K on a 32 bit system; for TICKS_HZ=10
+ * Uses ~280K on a 64 bits system and ~140K on a 32 bit system; for TICKS_HZ=10
  * holds ~ 30 min in the first hash/wheel and ~233h in the first two.
  * More perfomant arrangement: 16, 8, 8 (but eats 1 MB on a 64 bit system, and
  *  512K on a 32 bit one). For TICKS_HZ=10 it holds almost 2h in the
  *  first hash/wheel and ~460h in the first two.
  */
- */
 /*@{ */
 
 #define H0_BITS 14