Documentation/features-refresh.sh: Only sed the beginning "arch" of ARCH_DIR

It should only sed the beginning "arch" of ARCH_DIR in features-refresh.sh,
otherwise loongarch is recognized as loong, that is not what we want.

Fixes: be99f610a110 ("Documentation/features: Add script that refreshes the arch support status files in place")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Link: https://lore.kernel.org/r/1670156327-9631-2-git-send-email-yangtiezhu@loongson.cn
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/features: Add script that refreshes the arch support status files in place

Provides the script:

Documentation/features/scripts/features-refresh.sh

which operates on the arch-support.txt files and refreshes them in place.

This way [1],

"[...] we soft- decouple the refreshing of the entries from the
introduction of the features, while still making it all easy to
keep sync and to extend."

[1] http://lkml.kernel.org/r/20180328122211.GA25420@andrea

Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>