cmd/pprof: remove -show_bytes from command line help

This flag no longer exists.  It has been replaced with -unit=byte.

Change-Id: Iff9fc501f2c666067c9b1948c4623c8e3adddb8d
Reviewed-on: https://go-review.googlesource.com/1287
Reviewed-by: Russ Cox <rsc@golang.org>