x/website: update go list -m documentation for workspace mode in ref/mod

[perillo]

The documentation of go list -m currently says:
"If no arguments are specified, the main module is listed."

This is incorrect, since, when workspace mode is enabled, go list -m lists all the active main modules.

Here is an example:
$ go list -m -json

{
	"Path": "xxx",
	"Main": true,
	"Dir": "/tmp/src/xxx",
	"GoMod": "/tmp/src/xxx/go.mod",
	"GoVersion": "1.18"
}
{
	"Path": "yyy",
	"Main": true,
	"Dir": "/tmp/src/yyy",
	"GoMod": "/tmp/src/yyy/go.mod",
	"GoVersion": "1.18"
}
{
	"Path": "zzz",
	"Main": true,
	"Dir": "/tmp/src/zzz",
	"GoMod": "/tmp/src/zzz/go.mod",
	"GoVersion": "1.18"
}

[dr2chase]

@hyangah not sure if this is yours or @dmitshur, but I this looks reasonable to me.

[dmitshur]

Since this involves the go command's new workspace mode, I'll CC @matloob to take a look.