Pprof usage message includes "%" symbols. Misuse of Fprintf caused
the message to be interpreted as a format string and corrupted the usage
message.

Change-Id: I4732b491e2368cff9fdbfe070c125228d6f506fd
Reviewed-on: https://go-review.googlesource.com/115595Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>