docs: Update the react client install instructions (#1584)
docs: Update the react client install instructions (#1584)

The install instructions didn't previously add the
`unleash-proxy-client` package (presuming it'd be implicitly installed
by `npm`). This doesn't seem to be the case, so we'll list it explicitly.
Unleash/unleash,Unleash/unleash,Unleash/unleash,Unleash/unleash