tracing/doc: Fix table format in histogram code

The addition of the buckets conversion for the histogram code, updated the
documentation table of available conversions, but did not update the format
to accommodate the extra size needed to cover the description.

Link: https://lkml.kernel.org/r/20210823100007.71ce2ba9@oasis.local.home

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Tested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Documentation/trace/histogram.rst

@@ -70,16 +70,16 @@ Documentation written by Tom Zanussi
   modified by appending any of the following modifiers to the field
   name:
 
-	=========== ==========================================
-        .hex        display a number as a hex value
-	.sym        display an address as a symbol
-	.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
+	=============  =================================================
+        .hex           display a number as a hex value
+	.sym           display an address as a symbol
+	.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
 	.buckets=size  display grouping of values rather than raw number
-	.usecs      display a common_timestamp in microseconds
-	=========== ==========================================
+	.usecs         display a common_timestamp in microseconds
+	=============  =================================================
 
   Note that in general the semantics of a given field aren't
   interpreted when applying a modifier to it, but there are some