Use npm for Deno Astro installation

Keep the package README consistent with the manual and Vite's dependency
resolution by installing the Astro integration from npm under Deno.

#936 (comment)

Assisted-by: Codex:gpt-5.6-sol

Author: dahlia

packages/astro/README.md

@@ -170,7 +170,7 @@ Installation
 ------------
 
 ~~~~ sh
-deno add jsr:@fedify/astro  # Deno
+deno add npm:@fedify/astro  # Deno
 npm  add     @fedify/astro  # npm
 pnpm add     @fedify/astro  # pnpm
 yarn add     @fedify/astro  # Yarn