Update generate-image-metadata.mdx (#52230)

I read the PR checklist, but this is such a small PR that I feel it's
self-explanatory.

I believe the wrong function name (`generateMetadata`) is used on this
page, and it was meant to be `generateImageMetadata`.

If not... apologies 😅

docs/02-app/02-api-reference/04-functions/generate-image-metadata.mdx

@@ -13,7 +13,7 @@ You can use `generateImageMetadata` to generate different versions of one image
 
 ## Parameters
 
-`generateMetadata` function accepts the following parameters:
+`generateImageMetadata` function accepts the following parameters:
 
 #### `params` (optional)