Showdown package is moved to `markdown` package

This [showdown package](https://docs.meteor.com/#/full/showdown) is moved to markdown.

I typed in terminal 
```
meteor add showdown
```
then

```
showdown without a version constraint has already been added.
                                              
showdown: Moved to the 'markdown' package     
```

So I change full-api.
You should also remove this comment.