Doc:pps: Fix typo in pps.txt

This patch fix a spelling typo in Documentation/pps/pps.txt

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Acked-by: Rodolfo Giometti <giometti@enneenne.com>
[jc: did s/into/in the/ on the same line while we were there]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>