boards: st: nucleo_h7s3l8: Fix format in doc/index.rst

Fix wrong heading type in index.rst introduced with PR zephyrproject-rtos#95388

Signed-off-by: Thomas Decker <decker@jb-lighting.de>

Author: TD-JBL

boards/st/nucleo_h7s3l8/doc/index.rst

@@ -184,7 +184,7 @@ In order to test backup SRAM you may want to disconnect VBAT from VDD. You can
 do it by removing ``SB13`` jumper on the back side of the board.
 
 FDCAN
-=====
+-----
 
 The Nucleo H7S3L8 board does not have any onboard CAN transceiver. In order to
 use the FDCAN bus on this board, an external CAN bus transceiver must be