Remove deprecated add! in the stdlib overview doc

See with current master. push! works fine like with any other collection.

```
julia> a = Set()
Set{Any}()

julia> add!(a, 1)
WARNING: add!(s::Set,x) is deprecated, use push!(s,x) instead.
 in add! at deprecated.jl:19
Set{Any}(1)
```
Fix erroneous inserted comma
Merge pull request #5718 from xhochy/patch-2

Remove deprecated add! in the stdlib overview doc