Fix minor typo in no-img-element.md (vercel#29027)

Fix minor typo: 

image optimization -> Image Optimization

errors/no-img-element.md

@@ -2,7 +2,7 @@
 
 ### Why This Error Occurred
 
-An HTML `<img>` element was used to display an image. For better performance and automatic image optimization, use Next.js' built-in image component instead.
+An HTML `<img>` element was used to display an image. For better performance and automatic Image Optimization, use Next.js' built-in image component instead.
 
 ### Possible Ways to Fix It