cmd/go: remove a duplicated word from 'go help mod graph'

[ferrmin]

For golang#46366

Change-Id: Ie9735027a3c4c0f4a604df30ca4d64dcdc62b45a
Reviewed-on: https://go-review.googlesource.com/c/go/+/334375
Trust: Bryan C. Mills [email protected]
Run-TryBot: Bryan C. Mills [email protected]
Reviewed-by: Jay Conrod [email protected]
TryBot-Result: Go Bot [email protected]

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