stat/distuv: Fix discrepancy between Src and Source about stat

[sbinet]

I can send a PR for this, but: should it be Src or Source ? :)

[sbinet]

in math/rand.Rand, the field is called src:

// A Rand is a source of random numbers.
type Rand struct {
    src Source

    // readVal contains remainder of 63-bit integer used for bytes
    // generation during most recent Read call.
    // It is saved so next Read call can start where the previous
    // one finished.
    readVal int64
    // readPos indicates the number of low-order bytes of readVal
    // that are still valid.
    readPos int8
}

[btracey]

Fixed by gonum/gonum#355