Show using Flux before BSON @load

To a novice, it is not obvious that `Flux` module must be in scope before one can `@load` a model saved with `BSON.@save`.
This small change makes that requirement explicit.

Author: JeffFessler

docs/src/saving.md

@@ -24,6 +24,8 @@ julia> @save "mymodel.bson" model
 Load it again:
 
 ```jldoctest saving
+julia> using Flux # must precede @load
+
 julia> using BSON: @load
 
 julia> @load "mymodel.bson" model