kbuild: doc: Update default INSTALL_MOD_DIR from extra to updates

The default INSTALL_MOD_DIR was changed from 'extra' to
'updates' in commit b74d7bb7 ("kbuild: Modify default
INSTALL_MOD_DIR from extra to updates").

This commit updates the documentation to align with the
latest kernel.

Fixes: b74d7bb7

 ("kbuild: Modify default INSTALL_MOD_DIR from extra to updates")
Signed-off-by: Mark-PK Tsai <mark-pk.tsai@mediatek.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>