runtime/pprof: fix resource leak in documentation

[rillig]

Does this issue reproduce with the latest release?

yes

What did you do?

I copied the example code from runtime/pprof into my project and used it. It worked well.
Today I wrote a test for that piece of code, and I could not delete the profiling file because it was still busy.
Adding the appropriate f.Close() call fixed the problem.

[gopherbot]

Change https://golang.org/cl/129195 mentions this issue: runtime/pprof: fix resource leak in documentation