cmd/go: use modern pprof flags in documentation

This change updates our mentions of pprof flags so that we now refer to
the modern flag names instead of the legacy ones.

Fixes golang#74336.

Change-Id: Ie8fbc7407178219b84ce5171fa5e94621129267e
Reviewed-on: https://go-review.googlesource.com/c/go/+/689095
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

Author: jitsu-net

src/cmd/go/alldocs.go

@@ -186,8 +186,8 @@ and flags that apply to the resulting test binary.
 
 Several of the flags control profiling and write an execution profile
 suitable for "go tool pprof"; run "go tool pprof -h" for more
-information. The --alloc_space, --alloc_objects, and --show_bytes
-options of pprof control how the information is presented.
+information. The -sample_index=alloc_space, -sample_index=alloc_objects,
+and -show_bytes options of pprof control how the information is presented.
 
 The following flags are recognized by the 'go test' command and
 control the execution of any test: