tracing: doc: Fix typo in ftrace histogram

The Tracing > Histogram page contains a typo in the field display
modifiers table.

Signed-off-by: Gabriele Monaco <[e-mail]>
Acked-by: Steven Rostedt (Google) <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/trace/histogram.rst

···
 	.usecs         display a common_timestamp in microseconds
         .percent       display a number of percentage value
         .graph         display a bar-graph of a value
-	.stacktrace    display as a stacktrace (must by a long[] type)
+	.stacktrace    display as a stacktrace (must be a long[] type)
 	=============  =================================================
 
   Note that in general the semantics of a given field aren't