Specify type of sourcemap file (#4630)

Author: ZuBB

src/platforms/javascript/common/sourcemaps/validating.mdx

@@ -12,4 +12,4 @@ You can also check these, in addition to the validation step:
 
 - Ensure that the URL prefix is correct for your files. This is easy to get wrong.
 - Upload the matching source maps for your minimized files.
-- Ensure that the minified files you have on your servers actually have references to your files.
+- Ensure that the minified files you have on your servers actually have references to your `.map` files.