docs: fix DFU subsections showing up in main TOC

docs/en/api-guides/dfu.rst

@@ -53,7 +53,7 @@ The necessary connections for the {IDF_TARGET_NAME}'s internal USB PHY (transcei
 .. _api_guide_dfu_build:
 
 Building the DFU Image
-======================
+----------------------
 
 The command below will create a DFU image named ``dfu.bin`` that is placed in the project's ``build`` directory::
 
@@ -65,7 +65,7 @@ The command below will create a DFU image named ``dfu.bin`` that is placed in th
 .. _api_guide_dfu_flash:
 
 Flashing the DFU Image
-======================
+----------------------
 
 The command below will download the DFU image into the {IDF_TARGET_NAME}::
 

docs/zh_CN/api-guides/dfu.rst

@@ -53,7 +53,7 @@ USB 连接
 .. _api_guide_dfu_build:
 
 构建 DFU 镜像
-======================
+----------------------
 
 可以通过运行以下命令构建 DFU 镜像，该命令会在工程的 ``build`` 目录下生成 ``dfu.bin`` 文件::
 
@@ -65,7 +65,7 @@ USB 连接
 .. _api_guide_dfu_flash:
 
 烧录 DFU 镜像
-====================================
+------------------------------------
 
 运行以下命令将 DFU 镜像下载到 {IDF_TARGET_NAME} 中::