update frontmatter injection link to match docs changes (#5736)

Co-authored-by: Erika <3019731+Princesseuh@users.noreply.github.com>
Co-authored-by: Chris Swithinbank <swithinbank@gmail.com>

packages/astro/src/core/errors/errors-data.ts

@@ -523,7 +523,7 @@ See https://docs.astro.build/en/guides/server-side-rendering/ for more informati
 	/**
 	 * @docs
 	 * @see
-	 * - [Frontmatter injection](https://docs.astro.build/en/guides/markdown-content/#example-injecting-frontmatter)
+	 * - [Modifying frontmatter programmatically](https://docs.astro.build/en/guides/markdown-content/#modifying-frontmatter-programmatically)
 	 * @description
 	 * A remark or rehype plugin attempted to inject invalid frontmatter. This occurs when "astro.frontmatter" is set to `null`, `undefined`, or an invalid JSON object.
 	 */
@@ -532,7 +532,7 @@ See https://docs.astro.build/en/guides/server-side-rendering/ for more informati
 		code: 6003,
 		message:
 			'A remark or rehype plugin attempted to inject invalid frontmatter. Ensure "astro.frontmatter" is set to a valid JSON object that is not `null` or `undefined`.',
-		hint: 'See the frontmatter injection docs https://docs.astro.build/en/guides/markdown-content/#example-injecting-frontmatter for more information.',
+		hint: 'See the frontmatter injection docs https://docs.astro.build/en/guides/markdown-content/#modifying-frontmatter-programmatically for more information.',
 	},
 	// Config Errors - 7xxx
 	UnknownConfigError: {