internal/fuzz: improve minimizing message

In order to know the actual number of bytes
of the entire corpus entry, the coordinator
would likely need to unmarshal the bytes and
tally up the length. That's more work than it
is worth, so this change just clarifies that
the printed # of bytes is the length of the
entire file, not just the entry itself.

Fixes #48989

Change-Id: I6fa0c0206a249cefdf6335040c560ec0c5a55b4a
Reviewed-on: https://go-review.googlesource.com/c/go/+/361414


Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>