tracing: Add Documentation for log2 modifier

Add a line for the log2 modifier, to keep it aligned with
tracing/README.

Link: http://lkml.kernel.org/r/a419028bccab155749a4b8702d5b97af75f1578f.1516069914.git.tom.zanussi@linux.intel.com

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Documentation/trace/histogram.txt

@@ -73,6 +73,7 @@
 	.sym-offset display an address as a symbol and offset
 	.syscall    display a syscall id as a system call name
 	.execname   display a common_pid as a program name
+	.log2       display log2 value rather than raw number
 
   Note that in general the semantics of a given field aren't
   interpreted when applying a modifier to it, but there are some