tracing/histogram: Fix documentation inline emphasis warning

This fixes the warning:

Documentation/trace/histogram.rst:1766: WARNING: Inline emphasis
start-string without end-string

The issue was caused by an unescaped '*' character.

Link: https://lore.kernel.org/all/[email protected]/T/#m77da47432f5cc6521d4294ffdb9621949cc35d04
Link: https://lkml.kernel.org/r/[email protected]

Fixes: 2d2f6d4 ("tracing/histogram: Document expression arithmetic and constants")
Reported-by: Stephen Rothwell <[email protected]>
Signed-off-by: Kalesh Singh <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>

Author: Kalesh Singh

Documentation/trace/histogram.rst

@@ -1764,7 +1764,7 @@ using the same key and variable from yet another event::
   # echo 'hist:key=pid:wakeupswitch_lat=$wakeup_lat+$switchtime_lat ...' >> event3/trigger
 
 Expressions support the use of addition, subtraction, multiplication and
-division operators (+-*/).
+division operators (+-\*/).
 
 Note that division by zero always returns -1.