website: fix header on configure page

When the config file header was reworked, an erroneous link was
included and placed so close to the header that it was rendered verbatim
in the final documentation page.

By adding an extra empty line in between the anchor link and the header,
this renders correctly.

website/content/docs/configure.mdx

@@ -36,6 +36,7 @@ Examples:
   config directory will be: `C:/packer.d/` and other values will not be checked.
 
 <a id="packer-s-config-file"></a>
+
 ## Packer's config file (deprecated)
 
 Packer can optionally read a JSON file for the end user to set core settings.