README.md: remove duplicate "to adhere"

Fixes: 844669a9 ("README.md: add guideline for header guards format")
Reported-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210614100151.123622-1-cohuck@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>