swagger API branch screenshots defined as string rather than array

[Dan Mosedale]

Looking at the v7 / experiments / responses section of https://htmlpreview.github.io/?https://github.com/mozilla/experimenter/blob/main/docs/experimenter/swagger-ui.html, we see this:

    "branches": [
      {
        "slug": "string",
        "ratio": 0,
        "features": "string",
        "description": "string",
        "screenshots": "string"
      }
    ],

However, screenshots actually returns an array of strings.

CC @jaredlockhart @sarahhjchung

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