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 <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
Trust: Jay Conrod <[email protected]>
Trust: Daniel Martí <[email protected]>
Run-TryBot: Jay Conrod <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: shuLhan

src/cmd/go/alldocs.go

@@ -46,7 +46,7 @@ marking the root of the main (current) module.
 The go.mod file format is described in detail at
 https://golang.org/ref/mod#go-mod-file.
 
-To create a new go.mod file, use 'go help init'. For details see
+To create a new go.mod file, use 'go mod init'. For details see
 'go help mod init' or https://golang.org/ref/mod#go-mod-init.
 
 To add missing module requirements or remove unneeded requirements,