Syncing your fork is always a good idea when you create a Pull Request.

How to sync your AzerothCore fork

Open your terminal and move to the local clone of your AzerothCore fork.

Switch to your master branch (or whatever branch you need to sync):

git checkout master

Then type:

git remote add upstream https://github.com/azerothcore/azerothcore-wotlk.git

git fetch upstream

git merge upstream/master

Then when you are ready to push the changes (e.g. after resolving merge conflicts, if any):

git push