cmd/go: fix documentation

The command "go tool pprof help" does not work:

    $ go tool pprof help
    open help: no such file or directory

The right command is "go tool pprof -h".

Change-Id: Icef5d4ab76774905854e46665ac1166d26d35f46
Reviewed-on: https://go-review.googlesource.com/10970Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/alldocs.go

@@ -1103,7 +1103,7 @@ The 'go test' command takes both flags that apply to 'go test' itself
 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 help" for more
+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.
 

src/cmd/go/test.go

@@ -107,7 +107,7 @@ The 'go test' command takes both flags that apply to 'go test' itself
 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 help" for more
+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.