docs: update markdown examples

README.md

@@ -48,7 +48,7 @@ Lookup the content-type associated with a file.
 
 ```js
 mime.lookup('json')             // 'application/json'
-mime.lookup('.md')              // 'text/x-markdown'
+mime.lookup('.md')              // 'text/markdown'
 mime.lookup('file.html')        // 'text/html'
 mime.lookup('folder/file.js')   // 'application/javascript'
 mime.lookup('folder/.htaccess') // false
@@ -81,7 +81,7 @@ mime.extension('application/octet-stream') // 'bin'
 Lookup the implied default charset of a content-type.
 
 ```js
-mime.charset('text/x-markdown') // 'UTF-8'
+mime.charset('text/markdown') // 'UTF-8'
 ```
 
 ### var type = mime.types[extension]