Invalid prop error during onboarding flow

[ryanml]

Describe the bug
A prop (nextRoute) for FirstTimeFlow is incorrectly typed as a function, when it should be a string.

Other examples of its correct use:
https://github.com/MetaMask/metamask-extension/blob/develop/ui/app/pages/first-time-flow/metametrics-opt-in/metametrics-opt-in.component.js#L9

https://github.com/MetaMask/metamask-extension/blob/develop/ui/app/pages/first-time-flow/select-action/select-action.component.js#L13

To Reproduce

1. Start a clean build of MetaMask
2. Click 'Let's get started', then go to create a password
3. Note the screenshot for the error in the console

Expected behavior
Prop should be typed as a string, no console errors

Screenshots

Browser details (please complete the following information):

• OS: [e.g. OS X, Windows]
• Browser [e.g. chrome, safari]
• MetaMask Version [e.g. 5.0.2]

Additional context
Add any other context about the problem here.