docs(examples/next): fix case insensitive import

Hey @lachlanjc, I noticed that Next.js example was failing on this case-insensitive import while reviewing #2085 :)

examples/next/pages/_app.tsx

@@ -1,7 +1,7 @@
 /** @jsxImportSource theme-ui */
 import { ThemeProvider } from 'theme-ui'
 
-import Header from '../components/header'
+import Header from '../components/Header'
 import { theme } from '../lib/theme'
 import type { AppProps } from 'next/app'