DOC: add missing periods in JSON section
Hello, I am new here and this is my first contribution (following the advice on how to start contributing from [here](https://blog.afeld.me/contributing-to-pandas-1a68692c4d47)). I just added 2 periods that were missing in the JSON section of the user guide.
Thanks for the suggestion, I'll make the changes immediately.
 Thanks @JohannaTrost 