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/+/202137Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/pprof/pprof.go

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