fix debug msg in Runnable#slow; closes #2952

Author: boneskull

lib/runnable.js

@@ -103,7 +103,7 @@ Runnable.prototype.slow = function (ms) {
   if (typeof ms === 'string') {
     ms = milliseconds(ms);
   }
-  debug('timeout %d', ms);
+  debug('slow %d', ms);
   this._slow = ms;
   return this;
 };