Fix the go:generate for `doc.go`

[Razvan Musaloiu-E.]

We currently have 35 occurrences of go:generate of the following type:

//go:generate go run $JIRI_ROOT/release/go/src/v.io/x/lib/cmdline/testdata/gendoc.go . -help

The $JIRI_ROOT doesn't exist anymore so all these need to be adjusted.

[Razvan Musaloiu-E.]

This was fixed in d75ba39.