add missing newline to UnionAll Vararg warning

[simeonschaub]

added in #38136. Currently multiple warnings are printed like this, which is probably not intended:

~ >>> julia-latest --depwarn=yes                                                                              ✓ 500
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.7.0-DEV.168 (2020-12-25)
 _/ |\__'_|_|_|\__'_|  |  Commit dedf290c99 (1 day old master)
|__/                   |

julia> Vararg{<:Any}; Vararg{<:Any}
WARNING: Wrapping `Vararg` directly in UnionAll is deprecated (wrap the tuple instead).WARNING: Wrapping `Vararg` directly in UnionAll is deprecated (wrap the tuple instead).Vararg{Any}

[simeonschaub]

linuxaarch64 failure is #38996