runtime/pprof: fix resource leak in documentation

Fixes #26970

Change-Id: I0f2695434a53550cf84f702e9d8d02a37448d396
Reviewed-on: https://go-review.googlesource.com/129195Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/pprof/pprof.go

@@ -28,6 +28,7 @@
 //            if err != nil {
 //                log.Fatal("could not create CPU profile: ", err)
 //            }
+//            defer f.Close()
 //            if err := pprof.StartCPUProfile(f); err != nil {
 //                log.Fatal("could not start CPU profile: ", err)
 //            }
@@ -41,11 +42,11 @@
 //            if err != nil {
 //                log.Fatal("could not create memory profile: ", err)
 //            }
+//            defer f.Close()
 //            runtime.GC() // get up-to-date statistics
 //            if err := pprof.WriteHeapProfile(f); err != nil {
 //                log.Fatal("could not write memory profile: ", err)
 //            }
-//            f.Close()
 //        }
 //    }
 //