Remove traling newline in Markdown code blocks

[Keluaa]

This PR makes OhMyREPL match exactly the default Markdown formatting of the stdlib when OhMyREPL.HIGHLIGHT_MARKDOWN[] == false.

Formatted code in REPL help is then a bit more compact, with the downside that it might be a tad harder to make the difference between the output of a code example and text.

Before:

julia> using Markdown

julia> md"""
       A
       ```julia
       function a() end
       ```
       B
       """
  A

  function a() end

  B

julia> using OhMyREPL

julia> md"""
       A
       ```julia
       function a() end
       ```
       B
       """
  A

  function a() end


  B

After:

julia> using Markdown, OhMyREPL

julia> md"""
       A
       ```julia
       function a() end
       ```
       B
       """
  A

  function a() end

  B