x86_64: fix wrong comment regarding set_fixmap()

The function name is set_fixmap(), not fixmap_set() as stated in the comment.

Also fix a typo, punctuation and lower/uppercase a bit.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>