Fix typos in blog-starter and blog-starter-typescript (#15807)

Co-authored-by: Nkodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

examples/blog-starter-typescript/components/alert.tsx

@@ -18,7 +18,7 @@ const Alert = ({ preview }: Props) => {
         <div className="py-2 text-center text-sm">
           {preview ? (
             <>
-              This is page is a preview.{' '}
+              This page is a preview.{' '}
               <a
                 href="/api/exit-preview"
                 className="underline hover:text-cyan duration-200 transition-colors"

examples/blog-starter/components/alert.js

@@ -14,7 +14,7 @@ export default function Alert({ preview }) {
         <div className="py-2 text-center text-sm">
           {preview ? (
             <>
-              This is page is a preview.{' '}
+              This page is a preview.{' '}
               <a
                 href="/api/exit-preview"
                 className="underline hover:text-cyan duration-200 transition-colors"