fix: change type of prospect field in prospect application

[GermainBergeron]

I changed the type of the prospect field in the Create a new application for an existing candidate or prospect. endpoint, it was a boolean, but the description is referring to "true" which is confusing

When using "prospect": true as a boolean value I kept getting a 201 Created which wouldn't add the application on my prospect. After further investigation I realized the server responded with a body of

{
  "errors": [
    {
      "message": "Not found"
    }
  ]
}

which was really confusing.

By switching to "prospect": "true" I was able to make everything work.

Like others I expected to be able to add the prospect pool and prospect pool stage when creating a prospect