Add missing comma to metadata example

I tried copying and pasting this example of gemspec metadata into a gem
and noticed that it had a syntax error because there's a missing comma.

specification-reference.md

@@ -286,7 +286,7 @@ next: /command-reference
   "homepage_uri"      => "https://bestgemever.example.io",
   "mailing_list_uri"  => "https://groups.example.com/bestgemever",
   "source_code_uri"   => "https://example.com/user/bestgemever",
-  "wiki_uri"          => "https://example.com/user/bestgemever/wiki"
+  "wiki_uri"          => "https://example.com/user/bestgemever/wiki",
   "funding_uri"       => "https://example.com/donate"
 }</pre>