cmd/go: fix documentation on how to create new go.mod file
The correct command to create new go.mod file should be 'go mod init',
not 'go help init'.
Change-Id: I1150621987d989997f8b75e6a13fe96423a11cf3
Reviewed-on: https://go-review.googlesource.com/c/go/+/305289 Reviewed-by: Jay Conrod <jayconrod@google.com> Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Trust: Jay Conrod <jayconrod@google.com>
Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>