[Images] Fix code block with a missing backtick (#18852)

Author: AOrps

src/content/docs/images/transform-images/make-responsive-images.mdx

@@ -134,10 +134,10 @@ Replace `https://example.com` with your Cloudflare zone where transformations ar
 
 Alternatively, you can enable client hints via HTTP by adding the following headers to your HTML page's response:
 
-``txt
+```txt
 critical-ch: sec-ch-viewport-width, sec-ch-dpr
 
 permissions-policy: ch-dpr=("https://example.com"), ch-viewport-width=("https://example.com")
 ```
 
-Replace `https://example.com` with your Cloudflare zone where transformations are enabled.
+Replace `https://example.com` with your Cloudflare zone where transformations are enabled.