【OpenHarmony开源贡献者计划2022】update zh-cn/device-dev/porting/porting-stm32f407-on-minisystem-eth.md.

删除多余符号；
Signed-off-by: N程笠 <2321508477@qq.com>
Signed-off-by: N程笠 <2321508477@qq.com>

zh-cn/device-dev/porting/porting-stm32f407-on-minisystem-eth.md

@@ -637,7 +637,7 @@ board_ld_flags  ：链接选项，与Makefile中的LDFLAGS变量对应。
    
        sources += [
            "Src/main.c",
-   +       "Src/dprintf.c",
+           "Src/dprintf.c",
            "Src/stm32f4xx_hal_msp.c",
            "Src/stm32f4xx_it.c",
            "Src/system_stm32f4xx.c",