Update nested-routes.md to use UTF-8 box drawing characters

[Kerrick]

Use UTF-8 box drawing characters so it looks like this:

/user/johnny/profile                   /user/johnny/posts 
┌──────────────────┐                  ┌──────────────────┐
│ User             │                  │ User             │
│ ┌──────────────┐ │                  │ ┌──────────────┐ │
│ │ Profile      │ │  ●────────────▶  │ │ Posts        │ │
│ │              │ │                  │ │              │ │
│ └──────────────┘ │                  │ └──────────────┘ │
└──────────────────┘                  └──────────────────┘

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[posva]

Thanks!