[HardwareTimer]Fix typo

Issue raised by mlundin on the forum: https://www.stm32duino.com/viewtopic.php?f=38&p=5264#p5264 Signed-off-by: Frederic Pillon <frederic.pillon@st.com>

Author: fpistm

cores/arduino/HardwareTimer.cpp

@@ -1248,7 +1248,7 @@ timer_index_t get_timer_index(TIM_TypeDef *instance)
 #endif
 #if defined(TIM22_BASE)
  if (instance == TIM22) {
- index = TIMER2_INDEX;
+ index = TIMER22_INDEX;
  }
 #endif
  return index;