Change Content-Type suggestion

[adamhurm]

Using {'Content-Type': 'application/json'} will result in "400 Bad Request: invalid header name" response.

Additionally, I noticed there was some confusion like this issue: #81 and this discord post. In both scenarios a user submitted a screenshot where the Custom Header field is empty. It was not an issue with Kubernetes Goat, but maybe we can clear up the confusion by auto-filling fields or adding additional instructions to the scenario hints.

I see that you're using HACKTOBERFEST-ACCEPTED label so feel free to do that to this PR too.

[mrintern]

Honestly the main confusion is when one does exactly what is in the kubernetes goat walkthrough, the flag is not received. The walkthrough must be the source of truth.

I'm referencing this image from the walkthrough. It seems the "Custom Header" section is blank yet the flag is recieved

[madhuakula]

Yes @mrintern definitely you are right 👍 . But if you use this and deploy in GCP GKE, may be for metadata you need custom headers like "Metadata-Flavor: Google" and also there are lot more attack surface is there. So I will just add this fix :)

Thanks again @adamhurm as always you are awesome 🙏