Install prdoc from Parity fork (#5625)

Prdoc is now published as a Parity fork under the
[`parity-prdoc`](https://crates.io/crates/parity-prdoc) crate after the
directions diverged from the ideas of the original creator (discussions
[here](https://github.com/paritytech/prdoc/pull/40) and
[here](https://github.com/paritytech/prdoc/issues/36)).
Now updating the install instructions here.

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Signed-off-by: Oliver Tale-Yazdi <[email protected]>