Added ::IO type specifier on writemime methods in repl.jl

I noticed that the `writemime` methods in repl.jl apparently doesn't use the correct interface, because it does not specify that `io` must be of type `IO`

```julia
# Changed
function writemime(io, ::MIME"text/plain", f::Function)
# To
function writemime(io::IO, ::MIME"text/plain", f::Function)
```