Update configuration.md to reflect change in luaTarget values

[lostfictions]

Hi, I noticed the changelog notes the default value for luaTarget is universal now, but this wasn't reflected in the docs. This PR fixes that!

[lolleko]

Good Catch!

However there should also be a short description for universal e.g.:

Specifies the Lua version you want to generate code for. Choosing universal makes TypeScriptToLua generate code compatible with all supported Lua targets.

[lostfictions]

@lolleko done

[lolleko]

Thanks!