Update cdn.md (docsifyjs#479)

SidVal · QingWei-Li · commit 604989ff04e9 · 2018-05-10T06:53:44.000+08:00
`#compressed-file` instead `#compressed-files`
diff --git a/docs/cdn.md b/docs/cdn.md
@@ -12,7 +12,7 @@ Recommended: [unpkg](//unpkg.com), which will reflect the latest version as soon
 <script src="//unpkg.com/docsify/lib/docsify.js"></script>
 ```
 
-Alternatively, use [compressed files](#compressed-files).
+Alternatively, use [compressed files](#compressed-file).
 
 ## Specific version
 
