docs/ABI: testing: aspeed-uart-routing: Escape asterisk

Escape asterisk symbols to fix the following warning:

"WARNING: Inline emphasis start-string without end-string"

Fixes: c6807970c3bc ("soc: aspeed: Add UART routing support")
Reported-by: Stephen Rothwell <[e-mail]>
Signed-off-by: Chia-Wei Wang <[e-mail]>
Signed-off-by: Joel Stanley <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Link: https://lore.kernel.org/r/[e-mail]'
Signed-off-by: Arnd Bergmann <[e-mail]>

Documentation/ABI/testing/sysfs-driver-aspeed-uart-routing

···
-What:		/sys/bus/platform/drivers/aspeed-uart-routing/*/uart*
+What:		/sys/bus/platform/drivers/aspeed-uart-routing/\*/uart\*
 Date:		September 2021
 Contact:	Oskar Senft <[e-mail]>
 		Chia-Wei Wang <[e-mail]>
···
 		depends on the selected file.
 
 		e.g.
-		cat /sys/bus/platform/drivers/aspeed-uart-routing/*.uart_routing/uart1
+		cat /sys/bus/platform/drivers/aspeed-uart-routing/\*.uart_routing/uart1
 		[io1] io2 io3 io4 uart2 uart3 uart4 io6
 
 		In this case, UART1 gets its input from IO1 (physical serial port 1).
···
 Users:		OpenBMC.  Proposed changes should be mailed to
 		[e-mail]
 
-What:		/sys/bus/platform/drivers/aspeed-uart-routing/*/io*
+What:		/sys/bus/platform/drivers/aspeed-uart-routing/\*/io\*
 Date:		September 2021
 Contact:	Oskar Senft <[e-mail]>
 		Chia-Wei Wang <[e-mail]>