doxygen: fixed a minor typo for uart doc

This error is found when building with doxygen 1.9.8, but not
detecetd on 1.9.1.

Signed-off-by: Chen Wang <[email protected]>

Author: unicornx

documentation/6.components/device-driver/uart/uart.md

@@ -86,7 +86,7 @@ There are three modes of uart data receiving and sending: interrupt mode, pollin
 
 The DMA (Direct Memory Access) transfer mode does not require the CPU to directly control the transfer, and does not have the process of reserving the scene and restoring the scene as they have in the interrupt processing mode. The DMA controller opens a path for directly transferring data to the RAM and the I/O device, which saves CPU resources to do other things. Using DMA transfer can continuously acquire or send a piece of information without taking up interrupts or delays, which is useful when communication is frequent or when large pieces of information are to be transmitted.
 
->RT_DEVICE_FLAG_STREAM: Stream mode is used to output a string to the serial terminal: when the output character is `"\n"` (corresponding to the hexadecimal value 0x0A), a ``\r"` is automatically output in front (corresponding to hexadecimal value is 0x0D).
+> RT_DEVICE_FLAG_STREAM: Stream mode is used to output a string to the serial terminal: when the output character is `"\n"` (corresponding to the hexadecimal value 0x0A), a `"\r"` is automatically output in front (corresponding to hexadecimal value is 0x0D).
 
 The stream mode `RT_DEVICE_FLAG_STREAM` can be used with the receive and send mode parameter with the "|" logic.