fix(template): rename long blog post to .mdx to preserve MDX comment

Follow-up to 3129d39: prettier was mangling `{/* truncate */}` into
`{/_ truncate _/}` because the file extension was `.md`, causing it to
be parsed as Markdown where `*` is emphasis. Renaming to `.mdx` lets
prettier preserve the MDX comment syntax intact.

Author: sserrata

…efault/blog/2019-05-29-long-blog-post.md …fault/blog/2019-05-29-long-blog-post.mdxpackages/create-docusaurus-openapi-docs/templates/default/blog/2019-05-29-long-blog-post.md renamed to packages/create-docusaurus-openapi-docs/templates/default/blog/2019-05-29-long-blog-post.mdx packages/create-docusaurus-openapi-docs/templates/default/blog/2019-05-29-long-blog-post.md renamed to packages/create-docusaurus-openapi-docs/templates/default/blog/2019-05-29-long-blog-post.mdx

@@ -9,7 +9,7 @@ This is the summary of a very long blog post,
 
 Use a `{/* truncate */}` comment to limit blog post size in the list view.
 
-{/_ truncate _/}
+{/* truncate */}
 
 Lorem ipsum dolor sit amet, consectetur adipiscing elit. Pellentesque elementum dignissim ultricies. Fusce rhoncus ipsum tempor eros aliquam consequat. Lorem ipsum dolor sit amet