Impact: fix typo in documentation

TPPROTO is misspelt in Documentation/tracepoints.txt
Kept me wondering what was wrong, when I was trying to add a new tracepoint
subsystem.
Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>