docs: Fix a typo in Documentation/RCU/stallwarn.rst

Add the missing ')' in the documentation.
Signed-off-by: Haocheng Xie <xiehaocheng.cn@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/RCU/stallwarn.rst

@@ -224,7 +224,7 @@ is the number that had executed since boot at the time that this CPU
 last noted the beginning of a grace period, which might be the current
 (stalled) grace period, or it might be some earlier grace period (for
 example, if the CPU might have been in dyntick-idle mode for an extended
-time period.  The number after the "/" is the number that have executed
+time period).  The number after the "/" is the number that have executed
 since boot until the current time.  If this latter number stays constant
 across repeated stall-warning messages, it is possible that RCU's softirq
 handlers are no longer able to execute on this CPU.  This can happen if