Fix typos in Documentation/filesystems/seq_file.txt

A couple of typos crept into the newly added document about the seq_file
interface.  This patch corrects those typos and simultaneously deletes
unnecessary trailing spaces.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>