CONTRIBUTING.md references 'content' directory which no longer exists

[clyne]

After merging #159 the CONTRIBUTING.md file is now out of sync with reality; it references the directory 'content' which has been renamed to 'folder'.

cc @jukent

[brian-rose]

Good catch @clyne. Actually the CONTRIBUTING.md was already wrong... the content moved over to the portal directory some time ago. But nobody noticed the discrepancy.

Just submitted a PR to fix this.

[jukent]

Oh thank you! That makes so much more sense now. I was just fixing the image path and missed that there were other updates that needed to be made.