Fixes a grammar mistake in lib/index.js

Author: jatenate

lib/index.js

@@ -64,7 +64,7 @@ GitStats.record = function (data, callback) {
     }
 
     if (typeof data.url !== "string" || !data.url) {
-        callback(new Error("Invalid url field. This commit is not recorded into the git-stats history since you didn't added the remote url. You can import the previous commits using the git-stats-importer tool."));
+        callback(new Error("Invalid url field. This commit is not recorded into the git-stats history since you haven't added the remote url. You can import the previous commits using the git-stats-importer tool."));
         return GitStats;
     }