Merge #1156

1156: Add correct overload for apply! in docs r=dhairyagandhi96 a=dhairyagandhi96

Maybe we should considering adding a `const` name that is better than `apply!` (or rename `apply!`) and export it, so folks can just overload `descriptive_apply_my_optimiser_rule!` rather than have to go to the sub-project `Flux.Optimise`?

Co-authored-by: Dhairya Gandhi <dhairya@juliacopmuting.com>

docs/src/training/optimisers.md

@@ -80,7 +80,7 @@ Momentum(eta::Real, rho::Real) = Momentum(eta, rho, IdDict())
 The `Momentum` type will act as our optimiser in this case. Notice that we have added all the parameters as fields, along with the velocity which we will use as our state dictionary. Each parameter in our models will get an entry in there. We can now define the rule applied when this optimiser is invoked.
 
 ```julia
-function apply!(o::Momentum, x, Δ)
+function Flux.Optimise.apply!(o::Momentum, x, Δ)
   η, ρ = o.eta, o.rho
   v = get!(o.velocity, x, zero(x))::typeof(x)
   @. v = ρ * v - η * Δ