Typo in browser_add-on_activity.md

The lines
 ```
  git config --global user.email "joannakl@cs.nyu.edu"
  git config --global user.name "JoannaKl"```<br>
  ```
should be
 ```
  git config --global user.email "joannakl@cs.nyu.edu"
  git config --global user.name "JoannaKl"
  ```