docs: update schema code in docs for new canonicalURL property

src/content/blog/how-to-add-an-estimated-reading-time.md

@@ -81,6 +81,7 @@ export const blogSchema = z
     tags: z.array(z.string()).default(["others"]),
     ogImage: z.string().optional(),
     description: z.string(),
+    canonicalURL: z.string().optional(),
     readingTime: z.string().optional(), // 👈🏻 readingTime frontmatter
   })
   .strict();