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>