runtime/pprof: update example comment to check error on Close

Programs should always check the error return of Close for a file opened
for writing. Update the example code in the comment to mention this.

Change-Id: I2ff6866ff1fe23b47c54268ac8e182210cc876c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/202137


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>