[Example] Use .jpg for images in blog-starter (#11176)

* Use .jpg for images

* Updated links

examples/blog-starter/_posts/dynamic-routing.md

@@ -1,13 +1,13 @@
 ---
 title: 'Dynamic Routing and Static Generation'
 excerpt: 'Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Praesent elementum facilisis leo vel fringilla est ullamcorper eget. At imperdiet dui accumsan sit amet nulla facilisi morbi tempus.'
-coverImage: '/assets/blog/dynamic-routing/cover.webp'
+coverImage: '/assets/blog/dynamic-routing/cover.jpg'
 date: '2020-03-16T05:35:07.322Z'
 author:
   name: JJ Kasper
   picture: '/assets/blog/authors/jj.jpeg'
 ogImage:
-  url: '/assets/blog/dynamic-routing/cover.webp'
+  url: '/assets/blog/dynamic-routing/cover.jpg'
 ---
 
 Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Praesent elementum facilisis leo vel fringilla est ullamcorper eget. At imperdiet dui accumsan sit amet nulla facilisi morbi tempus. Praesent elementum facilisis leo vel fringilla. Congue mauris rhoncus aenean vel. Egestas sed tempus urna et pharetra pharetra massa massa ultricies.

examples/blog-starter/_posts/preview.md

@@ -1,13 +1,13 @@
 ---
 title: 'Preview Mode for Static Generation'
 excerpt: 'Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Praesent elementum facilisis leo vel fringilla est ullamcorper eget. At imperdiet dui accumsan sit amet nulla facilisi morbi tempus.'
-coverImage: '/assets/blog/preview/cover.webp'
+coverImage: '/assets/blog/preview/cover.jpg'
 date: '2020-03-16T05:35:07.322Z'
 author:
   name: Joe Haddad
   picture: '/assets/blog/authors/joe.jpeg'
 ogImage:
-  url: '/assets/blog/preview/cover.webp'
+  url: '/assets/blog/preview/cover.jpg'
 ---
 
 Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Praesent elementum facilisis leo vel fringilla est ullamcorper eget. At imperdiet dui accumsan sit amet nulla facilisi morbi tempus. Praesent elementum facilisis leo vel fringilla. Congue mauris rhoncus aenean vel. Egestas sed tempus urna et pharetra pharetra massa massa ultricies.