Author: Stephen Bourget
MDL-38277 Lesson: Fix strings on activity report
http://git.moodle.org/gw?p=moodle.git;a=commit;h=9e84423b79b8774a2c8b3ffd87ddbe614e64d65c
http://github.com/moodle/moodle/commit/9e84423b79b8774a2c8b3ffd87ddbe614e64d65c
+ 2.9 en [attemptheader,mod_lesson]
+ 2.9 en [numberofcorrectanswersheader,mod_lesson]
+ 2.9 en [numberofpagesviewedheader,mod_lesson]