irq: Fix typo in tracepoint.xml

This patch fix a spelling typo found in DocBook/tracepoint.xml.
It is because the file was created from comments in source,
so I have to fix the typo in include/trace/events/irq.h

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>