[logging] fix rtt logging not using defined buffer index (#9133)

examples/platforms/utils/logging_rtt.c

@@ -167,7 +167,7 @@ void utilsLogRttOutput(otLogLevel aLogLevel, otLogRegion aLogRegion, const char
     logString[length++] = '\n';
 
     // Write user log to the RTT memory block.
-    SEGGER_RTT_WriteNoLock(0, logString, length);
+    SEGGER_RTT_WriteNoLock(LOG_RTT_BUFFER_INDEX, logString, length);
 
 exit:
     return;