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 <gmonaco@redhat.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20241003122334.44682-2-gmonaco@redhat.com

Author: glemco

Documentation/trace/histogram.rst

@@ -81,7 +81,7 @@ Documentation written by Tom Zanussi
 	.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