Reduce string duplication in autodisovery code

Author: kadamwhite

lib/autodiscovery.js

@@ -16,18 +16,20 @@ const parseLinkHeader = require( 'parse-link-header' );
  * @returns {String} The URL of the located API root
  */
 function locateAPIRootHeader( response ) {
+	// See https://developer.wordpress.org/rest-api/using-the-rest-api/discovery/
+	const rel = 'https://api.w.org/';
+
 	// Extract & parse the response link headers
 	const link = response.link || ( response.headers && response.headers.link );
 	const headers = parseLinkHeader( link );
 
-	// See https://developer.wordpress.org/rest-api/using-the-rest-api/discovery/
-	const apiHeader = headers && headers[ 'https://api.w.org/' ];
+	const apiHeader = headers && headers[ rel ];
 
 	if ( apiHeader && apiHeader.url ) {
 		return apiHeader.url;
 	}
 
-	throw new Error( 'No header link found with rel="https://api.w.org/"' );
+	throw new Error( `No header link found with rel="${ rel }"` );
 }
 
 module.exports = {