Update 3.1.0.md

I was trying 
Member:
  extensions:
    MyMemberExtension

And it didn't work then someone on IRC pointed that I need to put a '-' before values. So this works.
Member:
  extensions:
    - MyMemberExtension
Hope will help someone else.
Merge pull request #2107 from vikas-srivastava/patch-1

Update 3.1.0.md