Remove redundant "Content-Type" from example in the Private Sharing user-story

[Potherca]

I noticed that an example in the Private Sharing user-story contained the Content-Type header twice.

For the sake of readability, I thought it might be better to remove it, hence this MR.

[csarven]

Thank you. Please note that this repo contains information that is outdated - It is not maintained and we pull information out of here to be carried out elsewhere as necessary. Current work on specifications is under https://github.com/solid/specification and other repositories (some are linked from the README).