doc: Fix broken HTML directive

This commit adds the needed "<".

Signed-off-by: Paul E. McKenney <[e-mail]>

Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html

···
 The best that <tt>sync_sched_exp_handler()</tt> can do is to check
 for idle, on the off-chance that the CPU went idle while the IPI
 was in flight.
-If the CPU is idle, then tt>sync_sched_exp_handler()</tt> reports
+If the CPU is idle, then <tt>sync_sched_exp_handler()</tt> reports
 the quiescent state.
 
 <p>