doc: usage: Update the extension command title

Update the extension command title for consistency with other commands.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

doc/usage/cmd/extension.rst

 .. SPDX-License-Identifier: GPL-2.0+
 .. Copyright 2021, Kory Maincent <kory.maincent@bootlin.com>
 
-U-Boot extension board usage (CONFIG_EXTENSION)
-===============================================
+extension command
+=================
 
 Synopsis
 --------