<asm-generic/siginfo.h>: fix language in comments

Fix grammar and add an omitted word.

Link: http://lkml.kernel.org/r/1a5a021c-0207-f793-7f07-addca26772d5@infradead.org
Fixes: f9886bc5

 ("signal: Document the strange si_codes used by ptrace event stops")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>